When the event starts and you've partnered up with someone, you'll be given a few tokens to help jump-start the event!

Tap Build below the partner you'd like to use tokens with, and your current progress will appear. At the bottom of the page, you'll see a button to 'Use Tokens.'  The button next to it will cycle through the amount of tokens you'd like to use, such as +1, +5, +10, +20, etc. Once you've decided on the amount, tap "Use Tokens" to add to the progress.