Are we talking about your own forum, or are you just trying to rank a post that’s already up on some established forum?
If it’s yours, I wouldn’t bother with indexers or tossing it into GSC—or any Google products, for that matter—unless you’re cool with leaving footprints for Google to sniff out. Your best move is setting up a 2-tier link campaign with something like RankerX or GSA SER. Personally, I’d go with GSA since I already own it. But if you don’t have it, give their trial a spin or check out RankerX. Both are solid for this kind of gig.
If it’s not your forum, you might need a decent indexer like Speedy Indexer. That’s the one I last used to get my GSA links indexed—worked like a charm. Haven’t touched it this year, though; that was a few months back.