XG Firewall captive portal registration options

Hello, 

We need people connecting to the open Wi-Fi network to log into a captive portal to keep track of their web activity.

So far the configuration that is closest to the behavior I want to achieve is the one described in this article:

support.sophos.com/.../KB-000038037

I followed those steps and it worked well, the invited users are registered with the phone number with which they requested their credentials by SMS.

However, I would like an option where sms is not required. It could be that users, instead of giving the phone number, they give the email and receive the access credentials in their inbox. Or fill out a form and then no need to enter credentials on the login page.

Are there any instructions to achieve this?

Thanks in advanced.