I’m using FormValidation and a bootstrap stepper function.
This is my code function:

var StepperDocumenti = function () {
            // Elements

            var stepper;
            var form;
            var formSubmitButton;
            var formContinueButton;

            // Variables
            var stepperObj;
            var validations = [];

            // Private Functions
            var initStepper = function () {
                // Initialize Stepper
                stepperObj = new KTStepper(stepper);

                // Validation before going to next page
                stepperObj.on('kt.stepper.next', function (stepper) {
                    console.log('stepper.next ' + stepper.getCurrentStepIndex());

                    // Validate form before change stepper step
                    var validator = validations[stepper.getCurrentStepIndex() - 1]; // get validator for current step
                    
                    if (validator) {
                        validator.validate().then(function (status) {
                            console.log('validazione step: ' + status);

                            if (status == 'Valid') {
                                stepper.goNext();

                                KTUtil.scrollTop();
                            } else {
                                Swal.fire({
                                    text: "Alcuni campi obbligatori non sono stati compilati correttamente, controllare.",
                                    icon: "error",
                                    buttonsStyling: false,
                                    confirmButtonText: "OK!",
                                    customClass: {
                                        confirmButton: "btn btn-light"
                                    }
                                }).then(function () {
                                    KTUtil.scrollTop();
                                });
                            }
                        });
                    } else {
                        stepper.goNext();

                        KTUtil.scrollTop();
                    }
                });

                // Prev event
                stepperObj.on('kt.stepper.previous', function (stepper) {
                    console.log('stepper.previous');

                    stepper.goPrevious();
                    KTUtil.scrollTop();
                });
            }

            var handleForm = function () {
                formSubmitButton.addEventListener('click', function (e) {
                    // Validate form before change stepper step
                    var validator = validations[2]; // get validator for last form

                    validator.validate().then(function (status) {
                        console.log('validated form: ' + status);

                        if (status == 'Valid') {
                            // Prevent default button action
                            e.preventDefault();

                            // Disable button to avoid multiple click
                            formSubmitButton.disabled = true;

                            // Show loading indication
                            formSubmitButton.setAttribute('data-kt-indicator', 'on');


                            // Simulate form submission
                            setTimeout(function () {
                                // Hide loading indication
                                formSubmitButton.removeAttribute('data-kt-indicator');

                                // Enable button
                                formSubmitButton.disabled = false;


                                form.submit();


                            }, 1000);
                        } else {
                            Swal.fire({
                                text: "Alcuni campi obbligatori non sono stati compilati correttamente, controllare.",
                                icon: "error",
                                buttonsStyling: false,
                                confirmButtonText: "Ok!",
                                customClass: {
                                    confirmButton: "btn btn-light"
                                }
                            }).then(function () {
                                KTUtil.scrollTop();
                            });
                        }
                    });
                });

            }

            var initValidation = function () {
                // Init form validation rules. For more info check the FormValidation plugin's official documentation:https://formvalidation.io/
                // Step 1
                validations.push(FormValidation.formValidation(
                    form,
                    {
                        fields: {
                            'anno': {
                                validators: {
                                    notEmpty: {
                                        message: 'L'anno è obbligatorio'
                                    }
                                }
                            }
                        },
                        plugins: {
                            trigger: new FormValidation.plugins.Trigger(),
                            bootstrap: new FormValidation.plugins.Bootstrap5({
                                rowSelector: '.fv-row',
                                eleInvalidClass: '',
                                eleValidClass: ''
                            })
                        }
                    }
                ));

                // Step 2
                validations.push(FormValidation.formValidation(
                    form,
                    {
                        fields: {
                            'id_um': {
                                validators: {
                                    notEmpty: {
                                        enable: false,
                                        message: 'L'unità di misura è obbligatoria'
                                    }
                                }
                            }
                        },
                        plugins: {
                            trigger: new FormValidation.plugins.Trigger(),
                            // Bootstrap Framework Integration
                            bootstrap: new FormValidation.plugins.Bootstrap5({
                                rowSelector: '.fv-row',
                                eleInvalidClass: '',
                                eleValidClass: ''
                            })
                        }
                    }
                ));
            }


            return {
                // Public Functions
                init: function () {
                    stepper = document.querySelector('#documenti_stepper');

                    if (!stepper) {
                        return;
                    }

                    form = stepper.querySelector('#form_documenti');
                    formSubmitButton = stepper.querySelector('[data-kt-stepper-action="submit"]');
                    formContinueButton = stepper.querySelector('[data-kt-stepper-action="next"]');

                    initStepper();
                    initValidation();
                    handleForm();
                }
            };
        }();


        // On document ready
        KTUtil.onDOMContentLoaded(function () {
            StepperDocumenti.init();
        });

I want to add a control that, when the select “tipo_riga” changes, performs follow check and enables or disables validation of field “id_um”

var tipo = $('[name="tipo_riga"]').val();
if (tipo == 'D') {
   validator.disableValidator('id_um');
} else if (tipo == 'C') {
   validator.disableValidator('id_um');
}

But I can’t figure out how to insert this control. If I have to create an external function or if I can insert a control inside the function itself that controls the change of the select.