How can I test direct Steam invite/join using Spacewar?

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I figured out a way and wanted to post it in case anyone else needs help with this.

  1. Download and install Spacewar for the user who will be joining.
  2. Find, download and install Windows Sysinternals Process Explorer. ( Process Explorer - Windows Sysinternals | Microsoft Docs )
  3. Run Process Explorer and setup a Path filter for “Spacewar”.
  4. Re-send invitation from host to the other user.
  5. While Process Explorer is running on the client, click the Steam join request and let it launch Spacewar (don’t worry about it not launching your game).
  6. Let Spacewar finish loading, then exit the game.
  7. Bring up Process Explorer and research the list. Find the first entry where it actually invokes SteamworksExample.exe
  8. Look at the Properties of the entry. The command line should show, at the end, something like: +connect_lobby 109775243872202570
  9. Setup a batch file to launch your game, and add that to your command at the end: +connect_lobby 109775243872202570
  10. Launch your game using that batch file. UE4 will then invoke the Steam join and you will be able to test out your game! (OnSessionUserInviteAccepted and OnJoinSessionCompleted should execute).