To achieve what you want you need to edit this GPO :
Open Gpedit with Administrator rights
Go to Windows Components > Remote Desktop Services > Remote Desktop Session Host > Connections > Set rules for remote control of Remote Desktop Services user sessions
Once you are on "Set rules for remote control of Remote Desktop Services user sessions",
set this GPO at 2 : Full Control without user's permission
or at 4 : View Session without user's permission
(we believe that you probably want 2)
Then also modify the file "AppControl.ini" located in "C:\program files (x86)\tsplus\userdesktop\files
and add under [Security] this line: "sessionsmanager-showconsentprompt = no"
Once done, when you will click in the AdminTool> Sessions Manager on the "Remote Control" button,
TSplus will ask Windows to take control of the session without displaying a confirmation prompt to the user.
However, modifying the .ini is not enough, the GPO must also be configured
(and a reboot will be mandatory for the GPO to be taken into account).