how to display iframe in safari

Site URL: https://www.thorpe-hall.org/paypalbookingform

I have a form built with google apps script: https://script.google.com/a/macros/thorpe-hall.org/s/AKfycbzSjpbW8moBJ75Q82NEux6ezDTbFAEtGIVki4i0PcelrbxjveletfyYtZ1ic9rQ_k-Szg/exec.

I would like to display it in a code block as an iframe: eg:

<iframe src="https://script.google.com/a/macros/thorpe-hall.org/s/AKfycbywQJsGAqumlhdBvw0AlYy6b2fFfqKy8cn8wgY0FYGCURQZApeVCnU2ZlxiUFyxKaTWAg/exec" >
  Loading ...
  </iframe>

This displays ok on MS edge and on Chrome but does not display at all in Safari, at least v11.

Interestingly, the parent div does display, just not the form.

Also, I have another a form, this one created using google forms, and included in a code block as an iframe:

<iframe

src="https://docs.google.com/forms/d/e/1FAIpQLScBeUGhrtxNNqq4F0FS0dWPwwTFMNAr6llRu-p343dYoWjUcg/viewform?embedded=true" >

</iframe>

and this DOES display fine in all three browsers mentioned above.

so it seems that my safari browser will display an iframe ok, but presumably only if it is setup ok. So my question is what needs to go into the header of my google apps script form to make it display ok.