There are a few options in form rules for displaying a message either when the record is opened, if the user is in a certain role, or based on data from fields in the form. It isn't perfect and JavaScript gives you more options, but it is a native solution. I'm sure you have seen these already though.
Maybe if you described what your intentions are with the popup someone would be able to direct you to a better solution. You could technically send the user to a code page and have that show your popup or information to the user in another way. Instead of a popup you could have the code page load with your desired information and have the user click okay and then be redirected back to where you want. This might work okay for this purpose or not, it just depends on what you are trying to do.