itch needs to allow monetization to its iframe
<iframe src = "/wm/example.htm" width = "600" height = "800" allow="monetization">
I sent an e-mail to Leaf about this.
Although this has already been announced elsewhere on the forum (https://itch.io/jam/the-web-monetization-challenge-2020/topic/1022867/web-moneti...) I thought it best to share it here as well that itch.io has added the "monetization" value to all HTML5 iframes.