Correct for strokeplay challenges - the post-game screen must be enabled in the game options.
This is a nice feature to stay in contact when you can't play vis-a-vis in a MP game. The invitees get a link to play the same game as you and after completion both get a comparable scorecard.
I assume that the OP talks about another, less transparent challenge, CTTH games.
The text "... challenges..." appears in some activity logs and displays CTTH scores from a short game course. With post-game screen enabled, you may share your score with friends and earn 10 XPs.
Does this answer your question, Ron?
Unfortunately, I've never been asked to answer such challenge - no idea how this would work.