You cannot create a '"mandatory form" for the users "after" they register into the community.
However, you can have a mandatory form being to be filled out before registration.
You need to create that form on your side. Then, using the Zapier13 app, as soon as the user fills out the form on your website/app/blog, Tribe will automatically send an invitation to the user to join the community.
Alternatively, you can add a link to the registration page of your community that redirects the users to the form. You can add it to the "signup rules" section explain here26.