It should automatically be added. Plus when you add miles to your yearly tracker it will be put on the streaker one too.
On my computer, when I look at the top left there's a drop down menu that might say "Select Event." On my cell phone if I click on the three lines at the top left, I can then click on "Select Event" to get to it.
ETA: On your cell phone you'll see all of the streaker challenges and the others have countdown clocks to when they will open.