Hook um_submit_form_{$mode}

um_submit_form_mode.php

<?php

/**
 * Hook: um_submit_form_{$mode}
 *
 * Type: action
 *
 * Description: Fires on the form submit.
 *
 * @example https://github.com/ultimatemember/ultimatemember/blob/master/includes/core/class-form.php#L589
 * @link    https://docs.ultimatemember.com/article/1281-umsubmitformmode
 *
 * @package um\core
 * @see     um\core\Form::form_init()
 * @since   2.0
 * @version 3.0
 *
 * @param array $post Post data.
 */
function my_submit_form_login( $post ) {
	// your code here.
}
$mode = 'login';
add_action( "um_submit_form_{$mode}", 'my_submit_form_login', 10, 1 );