You can enter a value in this field only if you choose "Add To Cart".

If you want the "Add To Cart" Summary Page (where users can see all the cart items, quantity, amount etc) to have your WordPress theme's look & feel, instead of a basic HTML page's look & feel, then create a WordPress page with the merge tag %%DAPCARTSUMMARY%% in the page body, save it, and put the URL of that page here.

Enter the full URL to the page.