How can I build a push notification subscriber list?

I have one signal set up but I was hoping to find some sort of modal feature ( something that hides the main page and appears over like a box that blurs the background) that I can activate so it appears over my lead magnet on my website, so that when visitors have to click to subscribe to get access to the lead magnet.

I see push engage has something like that but its kinda pricey. I intend to use propeller ads to run the ads to my website in order to build my list. Please let me know if I am on the right track or if there is a better option for the modal feature or using propeller ads as a traffic source.