I am trying to apply a custom filter on a lookup attribute.
The fetchXML returns correct values then why addCustomFilter is not working? I have turned off the related records filtering as well.

I have tried everything. The function fires up and doesn’t throw any errors.

function filterClassificationValues() {
    var fetchXML = "<filter type='and'><condition attribute='db_mailboxname' operator='eq' value='{guid of the record}' /></filter>";
               formContext.getControl("nameOfTheField").addCustomFilter(fetchXML, "entityName");
}

//this function is called On Load
function AddPreSearch(executionContext) {
    formContext = executionContext.getFormContext();
    formContext.getControl("nameOfTheField").addPreSearch(filterClassificationValues);
}