Add Custom validation to the field

Hello Team,

I have used this plugin and now I want to add custom validation on my field for the registration field.

I have sued the code which provided on the the site reference.

Please refer this link: https://docs.ultimatemember.com/article/94-apply-custom-validation-to-a-field

I used this code :

add_action(‘um_submit_form_errors_hook_’,’um_custom_validate_username’, 999, 1);
function um_custom_validate_username( $args ) {
global $ultimatemember;

if ( isset( $args[‘user_login’] ) && strstr( $args[‘user_login’], ‘admin’ ) ) {
$ultimatemember->form->add_error( ‘user_login’, ‘Your username must not contain the word admin.’ );
}
}

But it fires the error this:

“Uncaught Error: Call to a member function add_error() ”

So, How can I used this can you please explicit me in brif. And please give me the solution that how can I add the custom validation on my form field.

Thanks,
Riddhi Mehta