Shoutbox

What you saw was not a popup, it was a notification window like any other notification in Messenger, and it's for a contest organised this summer for all Plus! users to let them win prizes. We do that every year as a way to thank our users and do some promotion in the meantime. It has nothing to do with the sponsor or anything else, it's just a game of luck and you're free to participate or not, it won't ask any information from you unless you win anything.