Render external URL, from controller, in the same iframe

This is an embedded app in Shopify. I need to render a URL that allows the user to confirm a charge to their store. The URL is provided by Shopify, and I just need to render it. However, when I try with either of these options, it just hangs and the URL is never rendered.. BUT if I just copy and paste the URL into the browser it renders fine..

I tried with:

render @created_charge.confirmation_url
render :js => @created_charge.confirmation_url
redirect_to @created_charge.confirmation_url

And they didn't work.

The URL that needs to be rendered, looks like this:

http://ift.tt/1LELaKx
/confirm_recurring_application_charge?signature=XXXXX%2FYkHw%3D%3D--
9027c1eb56aXXXXXXXXXXXXX