Product Block Button URL

Site URL: https://www.realestateanalyzers.com/buy-now

I am working on building a pricing table through a 3rd party. I have express checkout enabled so that when I add a product block button and someone clicks the button it automatically adds that item to the cart and takes them to checkout. The 3rd party allows me to add a button with a url that I would like to replace the product block button since it looks nicer and is embedded into the pricing table. I am trying to figure out what url I need to enter within the 3rd parties website that will allow me to do this.

Hope this makes sense.

Thanks!

Dan