I personally think this request type is a "no-goer". The only thing you can do is select a "System" as the line item and add it to the request from "Existing assignments".
If you are using a LDAP connector to ensure the user data is populated and maintained correctly globally, then this request type is slightly wasted.
Best thing to do is use the "Change User" request type to ensure correct changes are made to the user profile, both in terms of access assignments and user details (if LDAP is not governing the content and the fields are modifiable).