"Mount&Blade Warband has not been installed on this computer."

[King Yngvar]

I'm getting the following error when trying to install NW now:
"Mount&Blade Warband has not been installed on this computer." - Yes it has.

Yes, there have been a lot of similar topics but I couldn't find a single one with a solution. Is there a way to just link it to steam via the serial key?

EDIT: I searched in the wrong place. A simple Google search for "add cd key to steam" revealed the solution.