If you’re asking if Discourse can use a subfolder instead of a subdomain, it’s possible, but not recommended:
However, from your roblox topic:
As a Roblox developer, it is currently too hard to return to the roblox homepage , I have to use my Stream Deck or type in “roblox” at the URL bar, and I want quick access to the roblox homepage
Your issue seems to be more like there is no direct link in the Roblox forum that leads to their main website, which has nothing to do with Discourse itself, but with their own forum customization. They can add a link to their website anywhere on their forum if they want to.
If the problem that you’re trying to solve is navigation, then the solution is to add navigation to the theme, not move things around to different URLs (which is sounds like is what is being discussed in the topic you linked).
Someone should just add or create a theme component that links to whatever other roblox sites you want to get to.