For the April Champion Challenge, I went to the Jamba Juice on 4/25, 1300 9th avenue in the Sunset at 7pm and they were closed already. I also didn't see any Sonic team members around. The Jamba Juice was supposed to close at 7:30, but for whatever reason was closed early. Really disappointed, the first event I tried to attend, I went out of my way for nothing.
Maybe you're looking for Sonic Drive-Ins? https://www.sonicdrivein.com/
Edit: Hmmm... I Googled "Sonic April Champion Challenge" and all the references were to the drive-in chain.