Code? – TestFlight Review

I opened my Instagram app and was greeted with a message asking if I wanted to try a new beta version of Instagram. I attempted to say yes and was taken to the app store where it had me download testflight. However after doing that I have not seen where I was supposed to get a code to use for it. I'm not sure what the deal is. I've checked my email and nothing.
Review by D.L. White on TestFlight.