How do I test a local iframe?

I added a html page as one of the files to my project and I want to view it from the running project as an iframe.

in the URL field of the iframe object I tried putting "/myHtmlFile.html" but nothing happens.

if I try file://myHtmlFile.html I get an error: "Not allowed to load local resource:"

also local/myHtmlFile.html fails with a 404 error..

Is there a way to do this?

ALso, I just tried uploading everything to the same folder on a server but I still get permission errors:

Uncaught DOMException: Blocked a frame with origin "https://www.pigobo.com" from accessing a cross-origin frame.

I cant understand because both the c3 index.html and the myHtmlFile.html are sitting in the same folder..?!?? How can this be a CORS issue?

Cheers