HTTP and HTTPS requests have headers; there's no such thing as a request without headers. In particular, you must have at least a HOST header and a Content-Length header when performing a POST.
I don't know what you mean when you say "adding header tag"-- HTTP headers are not tags.
You might try dragging a working POST request from Fiddler's Web Sessions list to the Composer to see what a proper request looks like.
You might try sharing a screenshot of what exactly the Composer looks like for your non-working scenario.
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