Update engagement task status in nodejs

Not making much progress. Per Update an Engagement in the legacy api docs, engagement is required in the JSON body, so I tried adding an engagement with various info from the existing engagement and even an empty engagement section like this:

 const apiOptions = {
 path: '/engagements/v1/engagements/'+id,
 method: 'POST',
 body: {
 "engagement": {
 },
 "metadata": {
 "status": 'COMPLETED',
 }
 }
 }

But no matter what I try, the response is always: “HttpError: HTTP request failed” giving no clues to what is wrong with the request. So it seems to be a matter of empirical testing to guess what might be expected. But I have not found the magic incantation yet.

Phil