The best thing to do is to upload to the IF Archive. But you’d need to be able to honestly check this box. “I am not the author, but to the best of my knowledge the author is okay with this.”
If you can find a way to contact the author, the very best thing to do is to ask the author’s permission to upload the game to the IF Archive.
If that’s not possible, it’s worth considering whether any of the author’s other works are archived, or whether it was submitted for a comp/jam (not in this case, I think?)
If you don’t think that you can honestly check that box, linking to the Internet Archive may be your best alternative.