how extend signup form.php?

how extend signup form.php?

by Claudio Curci -
Number of replies: 0
Hello,
I should set as mandatory the "city" field of the user registration form.
I should add this line
$mform->addRule('city', 'Insert your city, 'required', null, 'client');
to the file
/login/signup_form.php

Is there any way to extend this class ("login_signup_form") without modifying the original file (which will then be overwritten with Moodle updates)?
Thanks!
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