Hook um_custom_field_validation_{$custom}

um_custom_field_validation_custom.php

<?php

/**
 * Hook: um_custom_field_validation_{$custom}
 *
 * Type: action
 *
 * Description: Apply custom validation for the field.
 *
 * @example
 * @link
 *
 * @package um\core
 * @see     um_submit_form_errors_hook_()
 * @since   2.0
 * @version 3.0
 *
 * @param string $key   Field key.
 * @param array  $field Field data.
 * @param array  $args  Form arguments.
 */
function my_custom_field_validation( $key, $field, $args ) {
	// your code here.
}
add_action( "um_custom_field_validation_{$custom}", 'my_custom_field_validation', 10, 3 );