How to enable the “register with your phone” button

1
Enable the anyone can register option in your WordPress dashboard (located on the  Settings >  General page):

2
Enable the  Allow visitors to your site to register with Clef option at the bottom of the Clef plugin settings page:

3
Visit your site's register page (i.e, yoursite.com /wp-login.php?action=register), and the Clef “Register with your phone” button will appear at the bottom: