Custom validation to a field in 2.0

We used to be able to easily add form errors based on custom validation using the um_submit_form_errors_hook_ and the ultimatemember global variable, as found here:
https://docs.ultimatemember.com/article/94-apply-custom-validation-to-a-field

How can we do that in 2.0? I have tried various methods of the UM class like getting form() and then using add_error($key, $string). But they don’t work.

Is there an example of how to do this in 2.0 somewhere?