I remember doing this - you create a .txt or .html file, put it on your server, and then point the rokbox to that like you would any other file.....you need to have embedded .css from memory to style the popup tho.
go here, and click on "free evliery worldwide" at the top of the page to see it in action...that's just a basic .html file.
www.vitaloximeters.co.uk/
I can get you the exact line of code i've used if your struggling to get it going, just holla
