Autologin in iframe on external site

Hi

I am trying to autologin a user through a iframe on an external site. The external site has a membership on my site and wants to offer my services (membersite with fitness-videos) through an iframe on their site.

When the link with the autologin are loading in the iframe it loses won’t gain access and gets logged out.

Somebody who knows what causes this problem?

The page I need help with: [log in to see the link]