SSL on landing page

I am trying to connect a domain name (not a subdomain) to a Landing Page, but I cannot seem to make it resolve as https (thorugh an SSL on the domain).
My users are submittins sensitive information in the landing page and an SSL is very important for me

If I use the iframe option then the site becomes not responsive and looks pretty bad on a mobile device
Help please?

I have the same problem. Did you find a solution in the end?