Hook um_pre_{$mode}_shortcode

um_pre_mode_shortcode.php

<?php

/**
 * Hook: um_pre_{$mode}_shortcode
 *
 * Type: action
 *
 * Description: Fires before the form shortcode is loaded.
 *
 * @example https://github.com/ultimatemember/ultimatemember/blob/master/includes/core/class-password.php#L199
 * @example https://github.com/ultimatemember/ultimatemember/blob/master/includes/core/class-shortcodes.php#L752
 * @link    https://docs.ultimatemember.com/article/1198-umpremodeshortcode
 *
 * @package um\core
 * @see     um\core\Password::ultimatemember_password()
 * @see     um\core\Shortcodes::load()
 * @since   2.0
 * @version 3.0* Removed from the Password form.
 *
 * @param array $args Form shortcode arguments.
 */
function my_pre_login_shortcode() {
	// your code here.
}
$mode = 'login';
add_action( "um_pre_{$mode}_shortcode", 'my_pre_login_shortcode', 10, 1 );