Q: set thumbnail of a playlist?

Many 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 find 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 good monkey patch.

Please help.