Form verification callback function in Ci4

Thanks for the answer Digital_Wolf. I have read the documents many times. I do standard form validation. but I failed to call the function. I could not find an example about this.

CI3 code:

Code:

$this->form_validation->set_rules('password', 'password', 'required|callback_check_database');

How can I call the "callback_check_database" function here with CI4?