Go to the game page, click "Edit game", go to the "Categories" tab and click "New Category".
@Laika_the_Spacedog That's not what they meant.
@Monkeh_ Category Extensions can be added by the series moderators, ask them.
You can submit a request for a category extension board here: https://www.speedrun.com/requestgame?series= Select Single Game and set the game name as 'Miitopia Category Extensions'. You will have to provide a video of someone running one of the extension categories.
@SpiderHako Oh, my bad. Thanks for pointing it out.
Just to point out we're only granting Category Extensions on boards with enough activity. It seems Miitopia have only few players, so there's no need for a Category Extensions board for now. Though you can add categories as "Misc" on the main board if you want to track them.
@PapaFry you need to be a moderator of the game
The edit game button will be under the mod sword dropdown 
Also please do not bump old threads
@Bleeb maybe it's because you don't have permission to edit the game since you are not it's moderator🙄 I literally said that "you need to be a moderator of the game"
It depends what you need help with. What about the leaderboard do you want edited?
see, I want my free world record so can I get a category that is so easy that I get a free wr in it?
See if the advancement is in the category extensions:
OMG TYSM, it does not have a speedrun for what I'm looking for but I have some ideas now so Thank you so much ur the best pear
It's best, also why can't you look at the leaderboard yourself
imagine trying to add a category to a niche game that is only modded by one person who mods 101 other games. lol
note to self: good luck with that.