You can choose to let your users register themselves with a phone number. They will then receive a text message containing the password.
Here is what it looks like from the user's perspective:
This is the portal landing page where the user can enter his phone number and optional fields like his or her name.
Behind the scenes, when clicking the button, an API call will be made to generate the credentials for this user and send a text message containing the password to the provided phone number.
Then by going back and choosing the login button, he can fill in the form with his phone number and the password he or she received.