Hello. Client side modifications like this requires Javascript which is an area of my expertise. I can do it exactly the way you want but I have to note my experience on such popups, they can be very annoying because they interrupt visitor's interaction with the site. I suggest to implement in a way that not annoys the user. Contact me from chat anytime.
I got a notification form Escrow about partial payments while applying for this project, there is no need for any negotiation about the payment method, I'll deliver the work and make it working on your site without requiring you to pay for it. Because your profile is assuring enough, I'll finish the work and activate it on the required site. We can talk about the payment later.