How is the UserName and Password passed to the remote
datamodule?

On the RemoteDataModule, drop a TDatabase and add an
automation procedure (use Edit | Add To Interface) for
login.  Make sure that the HandleShared property is True
on the TDatabase.

procedure Login(UserName, Password: WideString);
begin
  { DB = TDatabase }

  { Something unique between clients }
  DB.DatabaseName := UserName + 'DB';  
  DB.Params.Values['USER NAME'] := UserName;
  DB.Params.Values['PASSWORD'] := Password;
  DB.Open;
end;

After you have created the automation method on the
RemoteDataModule (using Edit | Add To Interface), you can
call that method using;

  RemoteServer1.AppServer.Login('USERNAME','PASSWORD');