Nintendo just released an update for Mario Kart 8 Deluxe that lets you use your Toy-Con Motorbike from the from the Nintendo Labo: Variety Kit as a controller in the game. After the update, you’ll see a mew Nintendo Labo icon on the title screen.
The controls function as you probably assumed: turning the handlebars changes direction and twisting the right handlebar accelerates. The Joy-Con buttons are used for items, brakes, and drifting. If you have access to them, up to four players can play at the same time with Toy-Con Motorbikes.