httpRequest POST file Upload multipart

I had the same challenges as you when I was tackling this - documentation wasn't really fleshing it out well enough. ISC Sales Engineer helped me work through it.

Here is what I have used with success to submit both an XML doc and PDF doc to a vendor along with two parameters associated with the request (ReportId and CustomerId.) Requires use of MIME Parts. I hope this helps you. I had to genericize some of the code to share but it is commented by me what each part does and how it pulls together. 

Note this assumes you're passing in a variable pRequest that is a message class that holds your data. Also I am running this on 2019.1, not 2018 so not sure of the differences when using things like %JSONImport (may be none but I don't know that for certain.)

        Set tURL = "fully_qualified_endpoint_url_here"
        
        // Instantiate reportId MIME Part
        Set reportId = ##class(%Net.MIMEPart).%New()
        // Define/Set the Content-Disposition header indicating how this MIME part is encoded and what it contains.
        // Final string looks like: form-data; name="reportId"
        S tContentDisp1 = "form-data; name="_$CHAR(34)_"reportId"_$CHAR(34)
        Do reportId.SetHeader("Content-Disposition", tContentDisp1)
        // Get the ReportId from the incoming Request (from BPL) and write to the MIME Part body.
        S tReportId = pRequest.ReportId
        Set reportId.Body = ##class(%GlobalCharacterStream).%New()
        Do reportId.Body.Write(tReportId)
        
        // Instantiate customerId MIME Part
        Set customerId = ##class(%Net.MIMEPart).%New()
        // Define/Set the Content-Disposition header indicating how this MIME part is encoded and what it contains.
        // Final string looks like: form-data; name="customerId"
        S tContentDisp2 = "form-data; name="_$CHAR(34)_"customerId"_$CHAR(34)
        Do customerId.SetHeader("Content-Disposition", tContentDisp2)
        // Get the CustomerId from the incoming Request (from BPL) and write to the MIME Part body.
        S tCustomerId = pRequest.CustomerId
        Set customerId.Body = ##class(%GlobalCharacterStream).%New()
        Do customerId.Body.Write(tCustomerId)
        
        // Instantiate file1 (XML Structured Doc) MIME Part
        Set file1 = ##class(%Net.MIMEPart).%New()
        // Define/Set the Content-Disposition header indicating how this MIME part is encoded and what it contains.
        // Final string looks like: form-data; name="file1"; filename="<pRequest.CaseNumber>.xml"
        S tXmlFileName = pRequest.CaseNumber_".xml"
        S tContentDisp3 = "form-data; name="_$CHAR(34)_"file1"_$CHAR(34)_"; filename="_$CHAR(34)_tXmlFileName_$CHAR(34)
        Do file1.SetHeader("Content-Disposition", tContentDisp3)
        // Get the XML as a Stream from the incoming Request (from BPL) and write to the MIME Part body.
        Set tStream = ##class(%GlobalCharacterStream).%New()
        Set tSC = pRequest.XmlDoc.OutputToLibraryStream(tStream)
        Set file1.Body = tStream
        Set file1.ContentType = "application/xml"
        
        // Instantiate file1 (PDF Report) MIME Part
        Set file2 = ##class(%Net.MIMEPart).%New()
        // Define/Set the Content-Disposition header indicating how this MIME part is encoded and what it contains.
        // Final string looks like: form-data; name="file1"; filename="<pRequest.CaseNumber>.xml"
        S tPdfFileName = pRequest.CaseNumber_".pdf"
        S tContentDisp4 = "form-data; name="_$CHAR(34)_"file2"_$CHAR(34)_"; filename="_$CHAR(34)_tPdfFileName_$CHAR(34)
        Do file2.SetHeader("Content-Disposition", tContentDisp4)
        // Get the PDF Stream from the incoming Request (from BPL) and write to the MIME Part body.
        Set file2.Body = pRequest.PdfDoc.Stream
        Set file2.ContentType = "application/pdf"
        
        // Package sub-MIME Parts into Root MIME Part
        Set rootMIME = ##class(%Net.MIMEPart).%New()
        Do rootMIME.Parts.Insert(reportId)
        Do rootMIME.Parts.Insert(customerId)
        Do rootMIME.Parts.Insert(file1)
        Do rootMIME.Parts.Insert(file2)
        
        // Write out Root MIME Element (containing sub-MIME parts) to HTTP Request Body.
        Set writer = ##class(%Net.MIMEWriter).%New()
        Set sc = writer.OutputToStream(..%HttpRequest.EntityBody)
        if $$$ISERR(sc) {do $SYSTEM.Status.DisplayError(sc) Quit}
        Set sc = writer.WriteMIMEBody(rootMIME)
        if $$$ISERR(sc) {do $SYSTEM.Status.DisplayError(sc) Quit}
        
        // Set the HTTP Request Headers
        // Specify the Authorization header containing the OAuth2 Bearer Access Token.
        Set tToken = "set your token here or pull from wherever"
        Set tSC = ..%HttpRequest.SetHeader("Authorization","Bearer "_tToken)
        // Specify the Content-Type and Root MIME Part Boundary (required for multipart/form-data encoding.)
        Set tContentType = "multipart/form-data; boundary="_rootMIME.Boundary
        Set tSC = ..%HttpRequest.SetHeader("Content-Type",tContentType)

        // Call SendFormDataArray method in the adapter to execute POST. Response contained in tHttpResponse
        Set tSC=..Adapter.SendFormDataArray(.tHttpResponse,"POST", ..%HttpRequest, "", "", tURL)
        
        // Validate that the call succeeded and returned a response. If not, throw error.
        If $$$ISERR(tSC)&&$IsObject(tHttpResponse)&&$IsObject(tHttpResponse.Data)&&tHttpResponse.Data.Size 
        {
            Set tSC = $$$ERROR($$$EnsErrGeneral,$$$StatusDisplayString(tSC)_":"_tHttpResponse.Data.Read())
        }
        Quit:$$$ISERR(tSC)

        If $IsObject(tHttpResponse)
        {
            // Instantiate the response object
            S pResponse = ##class(Sample.Messages.Response.VendorResponseMsgClass).%New()
            // Convert JSON Response Payload into a Response Object
            S tSC = pResponse.%JSONImport(tHttpResponse.Data)

        }