Dynamic Cards: URL can't be parsed?

Hi,

I’m testing out Ghost locally and I want to use dynamic cards. I want to use dynamic cards, so I clicked the plus sign on a new post to try and see if I could embed content from another site. Upon pasting the URL however, it did not work, as ghost told me that this URL https://www.groupon.com/merchant/signup couldn’t be parsed and thus couldn’t be embedded. Anyone know how this works?

Thanks!