The only way you can add a user from a second domain is being able to open a session with a domain admin account of this second domain.
If that is not possible at all:
- edit the AppControl.ini file manually (located at C:\Program Files (x86)\TSplus\UserDesktop\files)
- type your domain name without extension like in the example below (Domain AD: TSPLUS.DOM) :
appname=Microsoft Remote Desktop