As far as I know, you can't get a matrix key unless you purchase the game there.

I do know that with a lot of games, if you buy it on matrix, they will give you a steam key. But it doesn't work the other way around like you want it too.

Thx for the answer, actually I just want a DRM free game, and I believe matrix version is DRM free. I know that GoG also has this game there, but it was too late as I already bought the game here, and I don't want to buy another copy on GoG just to be able to launch the game without steam. If one day steam goes down, all of the games that I'd paid here might be lost as well, so DRM free if possible from now on.