Hi, to my understanding this depends on the way you let users register/sign on to your app.
If you create their account (from Administration module) using a microflow, you can set the userrole (from System Module) immediately upon creation of the account object.
For example, you could retrieve the needed userrole(s) through an xpath constraint using the names you gave the userroles and then couple those to your created account object as member System.Userroles --> set
As Desiree says, you need to build some process for that.
For getting the UserRoles you should do a retrieve like this
I am using forgot password module to sign up