To clarify, you would like the guest user to select the user role they need to have?
EDIT: As soon as I submitted this I realise you can just use an input reference set selector for this
You can do this by retrieving the User Role in a listview (making sure to exclude roles you don’t want them to select.
Wrap this list view in a data view that has a microflow that retrieves the currently logged in account.
You can to this with XPath [id = $currentUser]
Then set an on-click action which calls a microflow that associates the current account with the userrole.