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) :


[App1]

appname=Microsoft Remote Desktop

path=

startup=

cmdline=

groups=

users=TSPLUS\user

maximized=no

minimized=no

hide=no

all_users=no