The IResponse::AddHeader method adds an HTTP header to the HTTP response. For more information, see the AddHeader method of the Response object.
HRESULT AddHeader(
BSTR bstrHeaderName, // contains the name of the HTTP header
BSTR bstrHeaderValue // contains the header value
);
This method adds an HTTP header with a specified value; it also always adds a new HTTP header to the response. It will not replace an existing header of the same name. Once a header has been added, it cannot be removed.