Hello Yubi,
You are right they replaced it with a combo box which is a new feature of Mendix.
To still achieve the same result you can create a reference set selector with a page yourself by doing the following.
1. create a new pop up page with a listview with as source userroles:
2. Put on your new account page a reference set input selector and connect your new page to this input selector go for attribute name:
This should work another possibility is to check download an earlier version of mendix and create a new app and check it out how they did it there.
Hope this helps,
Good luck