I have found it finally after many many many trials. It is this:
FString sid = "2171b53ac6";
const FString PostPayload = "{\"query\": \"query{comments(streamId:\\\"" + sid + "\\\"){items{authorId}}}\"}";
It shouldn’t be so difficult. The error responses are not helpful. I had to make a wild guess that took me 3 hours to find alternative string formats.