You can open your RedOrchestra.ini (Steam/Steamapps/common/red orchestra/system/RedOrchestra.ini) with WordPad or any other text editor and search for [Engine.Input] and add a line below it.
The line should look like this:
key=command
If you want to bind the togglescreenshotmode to "T" write T=togglescreenshotmode.
I hope that works, because I