Open the config and add a new line with worldname: with the name of the world you want to add to the UHC
Then add 2 parameters below this line, radius (raduis of blocks within the center where players get TP) and maxplayers (maximum number of players). It should do somethink like this:
worldname:radius:500`maxplayers:8
Restart your server
Then go in game and join the world that you added !