POSTing an XML string

I need to POST a normal-everyday XML data string to a website.

I FAIL when I POST with WinInet using:

Code:

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string Headers="Content-Type: application/xml";
string POSTdata="<?xml version=\"1.0\" encoding=\"utf-8\"?><note><to>Webserver</to><from>Me</from><re>My XML data</re></note>";
HttpSendRequest(hReq, Headers.c_str(), (DWORD)-1, (LPVOID)POSTdata.c_str(), strlen(POSTdata.c_str()))

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I SUCCEED when I POST with IXMLHTTPRequest using:

Code:

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http->setRequestHeader("Content-Type", "application/xml");
http->send(POSTdata.c_str());

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I read that IXMLHTTPRequest encodes the XML to UTF-8:
send Method (IXMLHTTPRequest)

Quote:

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If the input type is an XML DOM object, the response is encoded according to the encoding attribute on the <? declaration in the document. If there is no XML declaration or encoding attribute, UTF-8 is assumed.

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But, I am not sure if WinInet can even encode to UTF-8:
Issue in sending HTTP Request with UTF-8 URI using WinINet

Quote:

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It would be very helpful if I can get any documentation stating that Windows APIs don't support UTF-8.

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For me though, here is why I am confused:
UTF-8 - Wikipedia, the free encyclopedia

Quote:

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The first 128 characters of Unicode, which correspond one-to-one with ASCII, are encoded using a single octet with the same binary value as ASCII, making valid ASCII text valid UTF-8-encoded Unicode as well.

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So, what is the difference?
Byte versus Byte, character versus character.
Shouldn't my XML string be the same in UTF-8 versus the original C++ string?

If the answer is: "Yes, they should be the same" - then, why does WinInet FAIL?
If the answer is: "No, they are actually different" - then, what does my XML look like in UTF-8?

Thanks

Note:
WinInet doesn't FAIL in making the POST.
It's just that the website rejects the POST as being incorrect data.

Wireshark · Go deep.
Then you can see what the resulting messages are "on the wire", and compare the "good" with the "bad".

Unfortunately, the website that I am POSTing to is HTTPS.

But that does give me an idea...
Maybe I can POST to a friend's website and see the data that way.

Thank-you for spurring my mind.