I have the following code which adds a custom field to checkout for a specific product, adds it to the new order page, send’s it in the new order email and shows on the thank you for your order page.

// Utility function: Delivery partner options (array)
function get_delivery_partner_options() {
    return array(
        ''  => __('---Please select your Delivery Partner', 'woocommerce'),
        '1' => __('Learners University College (LUC), Dubai', 'woocommerce' ),
        '2' => __('Elite Training Services (ECS), KSA ', 'woocommerce' ),
        '3' => __('MELI, KSA', 'woocommerce' ),
        '4' => __('KLD Management Training, Dubai', 'woocommerce' ),
    );
}

// Display "Delivery Partner" custom checkout field 
add_action( 'woocommerce_after_checkout_billing_form', 'my_custom_checkout_field', 10, 1 );
function my_custom_checkout_field( $checkout ) {
    
    $targeted_product_id = 6245; //Global Membership ID

    foreach( WC()->cart->get_cart() as $item ){
        if ( ! in_array($targeted_product_id, [$item['product_id'], $item['variation_id']]) ) {
            return; // Exit if there are other products in cart than the targeted one.
        }
    }

    echo '<div id="my_custom_checkout_field">
    <h2>' . __('Please select your Delivery Partner', 'woocommerce') . '</h2>';

    woocommerce_form_field( 'delivery_partner', array(
        'type'       => 'select',
        'required'   => 'true',
        'options'    => get_delivery_partner_options(),
        'class'      => array('my-field-class form-row-wide'),
        'label'      => __('Delivery Partner', 'woocommerce'),
    ), $checkout->get_value( 'delivery_partner' ));

    echo '</div>';
}

// Validate delivery partner field
add_action( 'woocommerce_checkout_process', 'custom_checkout_field_validation' );
function custom_checkout_field_validation() {
    $field_key = 'delivery_partner';
    if ( isset($_POST[$field_key]) && empty($_POST[$field_key]) ) {
        wc_add_notice( __('<strong>'.__('Delivery Partner', 'woocommerce').'</strong> is a required field.', 'woocommerce'),'error' );
    }
}

// Save the selected delivery partner as order custom metadata:
add_action( 'woocommerce_checkout_create_order', 'save_selected_delivery_partner' );
function save_selected_delivery_partner( $order ) {
    $field_key = 'delivery_partner';
    $options   = get_delivery_partner_options();
    if ( isset($_POST[$field_key]) && ! empty($_POST[$field_key]) ) {
        $order->update_meta_data( 'delivery_partner', esc_attr($options[$field_key]) );
    } 
}

// Display the custom field value on admin order pages after billing adress:
add_action( 'woocommerce_admin_order_data_after_billing_address', 'display_selected_delivery_partner_in_admin_orders', 10, 1 );
function display_selected_delivery_partner_in_admin_orders( $order ) {
    if ( $delivery_partner = $order->get_meta('delivery_partner') ) {
        echo '<p><strong>'.__('Delivery Partner', 'woocommerce').':</strong> ' . $delivery_partner . '</p>'; 
    }
}

// Display on customer orders and email notifications
add_filter( 'woocommerce_get_order_item_totals', 'display_delivery_on_order_item_totals', 10, 3 );
function display_delivery_on_order_item_totals( $total_rows, $order, $tax_display ) {
    if ( $delivery_partner = $order->get_meta('delivery_partner') ) {
        $new_total_rows = [];

        // Loop through order total rows
        foreach( $total_rows as $key => $values ) {
            // Inserting before payment method
            if( $key === 'payment_method' ) {
                $new_total_rows['gift_order'] = array(
                    'label' =>__('Delivery Partner', 'woocommerce') . ':',
                    'value' => $delivery_partner,
                );
            }
            $new_total_rows[$key] = $values;
        }
        return $new_total_rows;
    }
    return $total_rows;
}

Everything seems to be working fine apart from: if I use a 100% discounted coupon it doesn’t show up on the thank you for your order screen / email.

Also how can I add multiple product ID’s to have the same checkout field?