The capture must be a zipfile as you can get when calling /export on an existing capture.

[Experimental] It can be any file in HTTP Archive format, from any source (browser or any other tool)
The URL in the bowser at the end of the capture, it cannot always be guessed from the HAR file.
A screenshot of the rendered page.
The page rendered by the browser at the end of the capture, it is not in the HAR file.

By default, the capture is public. If you do not want that, untick the box at the top of the form.