The HeaderField property returns the name of the current
header field. Setting this property causes the control to determine
if that header field exists, and if it does, to update the
HeaderValue property with its value. This property can be used
to obtain the value of any header in the current message.
Note that the server must support the XTND XLST command in order
to retrieve the header value. If the command is not supported, the
HeaderValue property will return an empty string.