A new ticket raffle system will be implemented into Pokemon Go for the Go Fest and Safari Zone events coming up this summer. From within the app, you will find an Events section in which you can enter a drawing for a chance to purchase tickets. If you are selected, you have 48 hours to complete registration and purchase the tickets.

This system aims to give everyone a fair chance to get tickets, since they are limited and have sold out in minutes for past events. Tickets are valid only for the Trainer account registered during purchase. You may purchase additional tickets for other Trainers (must be Great Friends). All event ticket sales are final. Refunds and transfers will not be accepted. This feature will be released when the Pokémon GO Fest Chicago ticket drawing starts.

For full details and FAQ: https://niantic.helpshift.com/a/pokemon-go/?p=web&s=events&f=registering-for-live-events