When i use a iframe on a https site, modern browsers block the insecure content inside the frame.
Is there an attribute, http header or similiar, to allow loading insecure content in the frame?
This question is off-topic for the Let’s Encrypt forums.
And no, I can’t think of anything off the top of my head that lets you do that other than an explicit user interaction with the browser UI.
You should start nagging the other website to use https.