Welcome! If you mean adding it to [this list](
/help/box.php?c=white&t=ga...;s=fulllist&nc=1) that GMs can pick from when creating a new game, good question. I imagine a mod would need to make the change. I think you can send them a message via rMail in the top right corner.
But, you can still start a TD game. Potential players wouldn't be able to search for it by system name, but you could select another system, 'None', 'Freeform', (aren't those the same?) or 'Homebrew' and then type '(Tiny Dungeon)' so that the label appears in any Players Wanted post you make later. I've seen at least one other system (Godbound) that's not on the list above advertised this way.
Curiously, there are at least three games in my sticky list where I can click 'Game Info' and it says 'The game system is Godbound.', so maybe you can change the system after creating the game?