Lots of answers about using the Youtube Data API v3 to get the thumbnail of a playlist; and many answers for how to get or set the thumbnail of a video. But none about how to set the thumbnail of a playlist. The documentation shows nothing and no searches into the API documentation or Stack Overflow finds the question or its answers.
I tried using the thumbnails().set() method for setting thumbnails to videos, but that returns a permission denied error indicating that using playlist IDs in its request for video IDs is not a sufficient monkey patch.
Please help.