To enter and play the Stella's Lucky Elevator event from elevator shaft 20 (instead of starting at shaft 1), you can use an Express Ticket. You will forfeit any possible rewards from levels 1 to 19, but you will start closer to the end goal of elevator shaft 50.
You can only use an Express Ticket when first starting the event - you cannot use one once you have started the event with a normal Elevator Ticket.
You can sometimes collect them as rewards in different Events around the game, or you can purchase them in the in-game Shop while Stella's Lucky Elevator is live.
See also: What are Elevator Tickets?