I need to create an Amazon MWS POST Request in MS VBA.
I started by testing our credentials in Amazon MWS Scratchpad. They worked fine.
Then I created the code below from the Request Details tab of Scratchpad.

When I execute the procedure below the response is:

    <?xml version="1.0"?>
<ErrorResponse xmlns="http://mws.amazonservices.com/schema/Products/2011-10-01">
  <Error>
    <Type>Sender</Type>
    <Code>InvalidParameterValue</Code>
    <Message>Either Action or Operation query parameter must be present.</Message>
  </Error>
  <RequestID>d75ae9c1-2df5-46f9-8638-df3a179117c3</RequestID>
</ErrorResponse>

I’m stuck trying to interpret the error code received. Just not sure what I’m missing.
Any help would be greatly appreciated.

Private Sub AmazonPostRequest()
    Dim httpReq As MSXML2.XMLHTTP60
    Dim XMLdoc As MSXML2.DOMDocument60
    Dim POSTdata As String
    
    POSTdata = _
      "https://mws.amazonservices.com/Products/2011-10-01?" & _
      "&Action=GetMatchingProduct" & _
      "ASINList.ASIN.1=BXXXXXXXXB" & _
      "&AWSAccessKeyId=AXXXXXXXXXXXXXXXXXXA" & _
      "&MWSAuthToken=AXXXXXXXXXXX9" & _
      "&MarketplaceId=AXXXXXXXXXXXR" & _
      "&SellerId=AXXXXXXXXXXX9" & _
      "&SignatureMethod=HmacSHA256" & _
      "&SignatureVersion=2" & _
      "&Timestamp=2019-04-19T19%3A25%3A08Z" & _
      "&Version=2011-10-01" & _
      "&Signature=jXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXD"

    Set httpReq = New MSXML2.XMLHTTP60
    With httpReq
        .Open "POST", "https://mws.amazonservices.com/Products/2011-10-01", False
        .setRequestHeader "Host", "mws.amazonservices.com"
        .setRequestHeader "user-agent", "AmazonTest"
        .setRequestHeader "Content-Type", "text/xml"
        .send POSTdata
        
        'Put XML response in a DOMDocument for parsing
        
        Set XMLdoc = .responseXML
        MsgBox XMLdoc.XML
    End With    
End Sub