Simple Access setup

I need two subdomains that needs to be hidden for the world, however I’m in no need for very strict security. The people that needs to access those pages are not technical and not always the same, so I need to set it up so I can just put a sticker with the login on the phone if they get logged out. The one time pin code wont work as theres no mailbox to send it to.

Any good idea on how to do this? A solution could be to make a dummy google account and use that, but I get an error when I try to added the redirect URI (" domain must be added to the authorized domains list").