Debug conversion of HTML code to VBA code

General information for the business: Software developer
Kind of development: Customization of existing program
Description of requirements/functionality: I am out of my depth on a particular piece of code and am looking for a solution to an error that I am getting. The code should post a multipart request consisting of filename, mime type and the file stream to an api end point.

This is a cut down version of the code as it currently stands

Const STR_BOUNDARY As String = "3fbd04f5-b1ed-4060-99b9-fca7ff59c113"
Dim xmlHtp As MSXML2.XMLHTTP60, Access_Token As String, eurl As String
Dim baBuffer() As Byte, FileNum As Integer, PostData As String

FileNum = FreeFile()
Open "C:\Users\DrHardy\VStocking\Test2.pdf" For Binary Access Read As #FileNum
ReDim baBuffer(0 To LOF(FileNum) - 1) As Byte
Get FileNum, , baBuffer
PostData = StrConv(baBuffer, vbUnicode)
Close #FileNum

Posting = "--" & STR_BOUNDARY & vbCrLf & _
"Content-Disposition: form-data; name=""Test2""; filename=""C:\Users\DrHardy\VStocking\Test2.pdf""" & vbCrLf & _
"Content-Type: application/pdf" & vbCrLf & vbCrLf & PostData & vbCrLf & "--" & STR_BOUNDARY & "--"

Set xmlHtp = New MSXML2.XMLHTTP60
With xmlHtp
.Open "post", eurl, False
.setRequestHeader "Access-Token", Access_Token ' Token is only valid for 60 minutes
.setRequestHeader "Content-Type", "multipart/form-data ; boundary=" & STR_BOUNDARY
.send Posting
Debug.Print .ResponseText
End With

The output from the debug is {"code":"NO_FILE_CONTENT","message":"Must provide file body"}

I have based my solution on this post https://wqweto.wordpress.com/2011/07/12/vb6-using-wininet-to-post-binary-file/

If you Google the contents of the STR_BOUNDARY variable you will find many other similar solutions.

The documentation for the api has this code for using HTTP

POST /api/rest/v5/transientDocuments HTTP/1.1
Host: api.na1.echosign.com
Access-Token: MvyABjNotARealTokenHkYyi
Content-Type: multipart/form-data
Content-Disposition: form-data; name="File"; filename="MyPDF.pdf"



I believe that the problem lies with the implementation of

I have tried many alternatives to the solution shown above, if all the STR_BOUNDRY entries are removed the error is {"code":"INVALID_MULTIPART","message":"An invalid multipart was specified"}

I just need the solution to the error code.
Specific technologies required: HTML, VBA
OS requirements: Windows
Extra notes: