Sorry if it's a stupid question. I see a search box at the top of the page, but it seems it's for the game names etc. I want to post a question in this forum, but it might be previously asked and solved, so I want to make a keywords search first. Is it available?

There is no way to do that inside the site itself. But you can do a google search in the form of "speedrun.com forums {subject}", and you might find similar results.

It's nice to see people being smart enough to make sure their question has been addressed before

If searching forum posts were possible, they would be able to see if the question had already been asked. Otherwise, they have no quick way of knowing.

One quick fix for this particular issue is to change the "etc" to make it clear what can be searched for.

"Search for series, games, users, and marathons"

Reference: https://www.speedrun.com/post/mqili