Including hashed signature in HTTP header VB6

Hi Experts -
I have VB6 code that I use to send an XML string to customer URL.
Recently, the customer asked us to include a digital signature in the header.
I've created the signature successfully and it is returned below as sDigSig.

I am not confident of the way I include the signature in HTTP header.
I've been all over the internet and have failed to find how this should be coded.

Please see the line below:
    objXmlHttp.setRequestHeader sDigSig, objDom.xml
below.

I think i need it to say something like:
    objXmlHttp.setRequestHeader  "Content-Type", "hash", sDigSig
but as i mentioned - i am coming up empty w/ the way I'm searching.

Also - we prefer to send this transmission asynchronously (timing is a premium).
Is this still possible to do when using .setRequestHeader ?

Thank you
klheitz

        Set objDom = CreateObject("MSXML2.DOMDocument")
        Set objXmlHttp = CreateObject("MSXML2.XMLHTTP")
       
        objDom.async = True    '07.26.13 change back to true

        '02.20.14 - digital sig to embed in http header...
        Dim sDigSig As String
        Dim objSig As New DigitalSignature.ComClass1
       
       
        sDigSig = objSig.GetSignature(strXML)
        Set objSig = Nothing        
       
        objDom.loadXML

            strPost = CUSTOMER_URL
                                   
            objXmlHttp.Open "POST", strPost, False
           
           
'***** // commented out
'     XMLHTTP.Open "POST", pstrURL, False ' False - syncronous mode
'
'    XMLHTTP.setRequestHeader "Content-Type", "text/xml"
'
'    XMLHTTP.setRequestHeader "X-Username", strUserName
'
'**** end comments

    objXmlHttp.setRequestHeader sDigSig, objDom.xml
                       
    objXmlHttp.Send objDom.xml
           
     strRet = objXmlHttp.responseText