There are many configuration guides on the Cisco website with details about configuring RADIUS and TACACS+ on a Cisco Firepower Chassis Manager. See this link for the configuration guide for 2.0(1).
In this document, you can read the following comment:
|Remote User Role Policy||Controls what happens when a user attempts to log in and the remote authentication provider does not supply a user role with the authentication information:|
But… it’s very hard to find what attributes are needed to assign a user the administrator role.