>>52126518And once the credits roll, you can select the legendary from the bottom of the menu as usual and get a prompt that you can add it to your party.
The first time you do so - either with an empty slot or swapping out one of your mons into the box, it immediately starts a battle with it at Lv 72, and then it can be treated as a regular Pokemon you own.
If you have a link to reetardkun's full VOD or one of the early postgame parts when he fucks around in Area Zero, I can search out the footage that proves Koraidon can simultaneously be ridden AND be a party member.