Registration with Roles in Mobile Platform

Hi!

I'm developing with WebRatio Mobile Platform 8.9.6 and I'm using the Free Cloud Account.

I would like to develop a registration form with choice of role between "Organizer" and "Partecipant". As default, I have the two class "User" and "Role", and when I created the two roles in the project, automatically, the table Role was populated.

But now, what kind of field I have to add to registration form to successfully add a user with particular role?

Thank you.