HTTP Header for Iframe from http

Dear all,

I have the following configuration:
I have included in a webpage (http://192.168.55.yyy - backend real server)
an iframe like this:

`<iframe src="

The webpage is running fine at https, all hhtp requests are redirected by haproxy to https

What would be the correct setting for http-response set-header X-Frame-Options
or other http response settings in order to allow the http iframe in the https webpage?