Embedding free netlify web page into iframe

I am evaluating netlify and I have come across what I suspect is a limitation of the free usage.
We need a web site to be embedded into a page on our web server to allow internal users to easily content for our web application’s front page. I created a test app in netlify but can’t get it’s headers set so that it can be included in an iframe.

Is this a limitation of the free usage or do I need to fiddle some more with the _header file?

Problem solved.
Turns out you can’t (seem) to alter the headers by editing the _headers file, but you can by altering the netlify.toml file.

That’s unexpected. Are you sure your _headers file was in your build directory after build? netlify.toml read from root of repo; _headers read from root of build output directory (next to index.html)