If you want to challenge your friend in the game, you can go to Menu>Play With a Friend or go straight to the event by going to Events>Private Events>Booster Clash and tap "Create".
This will take you to the booster select screen. Now tap on the booster you wish to use and then tap the "Play" icon under it.
Once you’ve done this, you’ll be given a code for the event and the option to "Share Link". You can either tell someone the code or share it by use of the "Share Link" icon. Once you tap it, you’ll be given a list of options for how you’d like to share the link. Choose your option and the link will be shown in a message to your chosen opponent. Now simply tap to send the message.
Your opponent now needs to either click on the link, or take the code and go to Events>Private Events. They will then see at the top that there’s a box that says "Enter Game Code". They simply need to enter the code and tap the "Join" icon next to it.
If you beat your friend and win the match, you get to keep their booster! But if you lose, you lose your booster.
Please note that matches that are forfeited or canceled before the game ends will not give the winner their opponent’s booster.
If you are not yet connected as friends, you will need to send your friend an invite to the game. You can find more information here.
Have fun challenging your friends to a Booster Clash!