Custom Dialog box explaining login credentials

[MPach]

I was trying to post this on Enterprise Sites, but I didn't see place for that, however, I'm pretty sure ArcGIS Hub and Enterprise Sites are pretty similar if not that same thing. 

What I'd like to do is create a custom login dialog box that pops up when a user tries to access a Sites - website that I created. My users always forget the format for their username login and I would like the dialog box to display that so I don't have to respond every time they forget. Is anything like that possible? 

I really would be thankful if they found out what a password keeper was, but that is not very likely unfortunately. 😀

Something along the lines of this or anything else that might work.

Thanks, 

Mark