It's not the button you need to set up, Itch.io projects can receive donations by default. As Leaf pointed out above, you need to configure payments on your account. You can do that in the Payment Processors section of your account settings.
In unrelated news, next time you have a problem, please start a new topic rather than necro-ing a 9-months-old thread. I only noticed your post by pure chance. As you can see, the creator you replied to wasn't so lucky. :)