Constant Contact landing form integration with Shopify?

We were exploring to add a landing page to our shopify page. We noticed that there is an an embed code available for inline/popup forms which we are unable to find for landing pages. How can we embed landng pages into our shopify page on the website ?  Example: We tried to use iframe option since no embed code is avaiable for landing page on the website which was able to host the landing form correctly but when we try to use the "Link to a webpage" redirect button on the Thankyou Page it gave us an error "website refused to connect".