Ultimate Member - finding the default login form id https://wordpress.org/plugins/cbxuseronline/

gistfile1.txt

global $wpdb;
$args = array();
$default_login = $wpdb->get_var(
	"SELECT pm.post_id 
				FROM {$wpdb->postmeta} pm 
				LEFT JOIN {$wpdb->postmeta} pm2 ON( pm.post_id = pm2.post_id AND pm2.meta_key = '_um_core' )
				WHERE pm.meta_key = '_um_mode' AND 
					  pm.meta_value = 'login' AND 
					  pm2.meta_value = 'login' "
);

$args['form_id'] = $default_login;
$shortcode_attrs = '';
foreach ( $args as $key => $value ) {
	$shortcode_attrs .= " {$key}=\"{$value}\"";
}

echo do_shortcode( "[ultimatemember {$shortcode_attrs} /]");