You can still put "Trail of Cthulhu" in your campaign description text & it will come up if someone types that in the Matching Keywords field on the LFG search engine. That's the best way to find game titles that aren't in the drop-down, so make sure you put the title in the Description and Keywords of your campaign. I recommend to list it under "Other Games" in the drop-down , by the way, or another general category if it fits like "Story games" or "Old School Revival" depending on what kind of game it is. I don't think there is a "Horror" general category though, again, just use keywords that you want people to find it by and hopefully that will work for finding a group, as well as, essentially, alerting Roll20 to the fact that it is a game that's seeing recruitment and listings among the "other" games.