I want to get the Proof of Purchase, but i didn't buy the game on steam. I got an item for the Mann Co Store. I bought the orange box on XBOX 360 though. Is there a way I can get the Proof of Purchase?
To get the Proof of Purchase, you need to activate a PC retail version of the game, receive the game as a gift, or had TF2 before it went free to play. Unfortunately, your Xbox version won't count. By "activate", I'm referring to adding the CD-Key to your Steam account's library, which can be done by clicking "Add Game" in the bottom left corner of the Library tab:
From the TF2 Wiki