flexible_checkout_fields_validate_billing_test_validate

functions.php

<?php
add_action( 'flexible_checkout_fields_validate_billing_test_validate', 'flexible_checkout_fields_validate_billing_test_validate' );
function flexible_checkout_fields_validate_billing_test_validate( $value ) {
    if ( $value != '1' ) {
        wc_add_notice( 'Test validate must equals 1!', 'error' );
    }
}