I would like to add a pop-up as soon as I enter the site. I’m starting from scratch
Technically, you’ld fire one with Javascript I assume?
In practice, they are seen as annoying by most users.
Also; this is not really kirby-related I think?
This.
Edit: If you ask google this question, you will find tons of tutorials how to achieve that. But I’d really think twice if this is desirable.