How to add a key

You'll be able to add a key when the winner is selected. Or, if you prefer, just add or email them.

Entering manually at the end should work fine.

If the system continues to not accept your key you can either send it to the winner's email which will be given to you once the giveaway ended, or you can add the winner on Steam and give it to them via Steam chat.

Awesome, thank you all for the replies. I was entering it exactly how it gave me, are you not allowed to use dashes?

From observation, having "blank spaces" at the beginning or end will corrupt it. Often happens with copy/pasting.
Dashes are part of the actual key however