How strict do you need this disappearing be? There are session variables where you can store a small bit of data in the users browser session.
https://www.wix.com/velo/reference/wix-storage
look at the differences between local, session, and memory. Some users block these however so it wouldn’t work for them. Other possible solutions I can think of are much more complex such as storing a flag if you have logged in users. I would start here though.