POST a Multi-Part Form to Bamboo API

I suspect that at the very least your Content-Length: 520 will be wrong. That content length was only applicable to their example.

Anyway, I haven't written VB.Net in a long, long time, but from a quick test a modified version of this code works against one of my REST services, so it should work in your case, with perhaps some minor tweaking.

My test console project used .Net 4.6.1, but will likely run with some of the earlier .Net Frameworks.

Imports System.IO
Imports System.Net.Http

Module Module1

    Sub Main()
        Call UploadFileToWebsite(14, "no", "D:\Temp\file.pdf")
        Console.WriteLine("Please wait for a response from the server and then press a key to continue.")
        Console.ReadKey()
    End Sub

    Public Sub UploadFileToWebsite(category As Integer, share As String, file As String)
        Dim message = New HttpRequestMessage()
        Dim content = New MultipartFormDataContent()

        content.Add(New StringContent(category.ToString()), "category")
        content.Add(New StringContent(share), "share")

        Dim filestream = New FileStream(file, FileMode.Open)
        Dim fileName = System.IO.Path.GetFileName(file)

        content.Add(New StreamContent(filestream), "file", fileName)

        message.Method = HttpMethod.Post
        message.Content = content
        message.RequestUri = New Uri("https://api.bamboohr.com/api/gateway.php/company")

        Dim client = New HttpClient()
        client.SendAsync(message).ContinueWith(
            Sub(task)
                'do something with response
                If task.Result.IsSuccessStatusCode Then
                    Console.WriteLine("Uploaded OK.")
                Else
                    Console.WriteLine("Upload Failed.")
                End If
            End Sub)
    End Sub
End Module

On an unrelated note, you can also use vbCrLf instead of vbCr & vbLf.

Used the php example on their GitHub and copied it over to VB.NET. It's a little messy, but it works. Here is the relevant code:

Public Function sendRequestMPF(ByVal req As BambooHTTPRequest, ByVal fileLocation As String) As BambooHTTPResponse
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 Or SecurityProtocolType.Ssl3

        Dim request As HttpWebRequest = WebRequest.Create(req.url)
        request.Method = req.method
        request.Host = "api.bamboohr.com"

        Dim boundary = "----BambooHR-MultiPart-Mime-Boundary----"

        Try
            request.ContentType = "multipart/form-data; boundary=" + boundary
            request.ContentLength = req.contents.Length
        Catch ex As Exception

        End Try

        Dim iCount As Integer = req.headers.Count
        Dim key As String
        Dim keyvalue As String

        Dim i As Integer
        For i = 0 To iCount - 1
            key = req.headers.Keys(i)
            keyvalue = req.headers(i)
            request.Headers.Add(key, keyvalue)
        Next

        Dim enc As System.Text.UTF8Encoding = New System.Text.UTF8Encoding()
        Dim bytes() As Byte = {}
        Dim pdfBytes() As Byte = {}
        Dim lBytes() As Byte = {}

        Dim fBytes() As Byte = {}
        Dim s As New MemoryStream()

        If (req.contents.Length > 0) Then
            bytes = enc.GetBytes(req.contents)
            s.Write(bytes, 0, bytes.Length)

            pdfBytes = File.ReadAllBytes(fileLocation)
            s.Write(pdfBytes, 0, pdfBytes.Length)

            Dim postHeader = vbCrLf + vbCrLf + "--" + boundary + "--" + vbCrLf
            Dim postHeaderBytes() As Byte = enc.GetBytes(postHeader)
            lBytes = enc.GetBytes(postHeader)
            s.Write(postHeaderBytes, 0, postHeaderBytes.Length)

            fBytes = s.ToArray()
            request.ContentLength = fBytes.Length
        End If

        request.AllowAutoRedirect = False

        If Not basicAuthUsername.Equals("") Then
            Dim authInfo As String = basicAuthUsername + ":" + basicAuthPassword
            authInfo = Convert.ToBase64String(Encoding.Default.GetBytes(authInfo))
            request.Headers("Authorization") = "Basic " + authInfo
        End If

        If req.contents.Length > 0 Then
            Dim outBound As Stream = request.GetRequestStream()
            outBound.Write(fBytes, 0, fBytes.Length)
        End If

        Dim resp As BambooHTTPResponse
        Try
            Dim webresponse As HttpWebResponse = request.GetResponse()
            resp = New BambooHTTPResponse(webresponse)
            resp.responseCode = webresponse.StatusCode
            resp.headers = webresponse.Headers
        Catch e As WebException
            Console.WriteLine(e.Message)
            If (e.Status = WebExceptionStatus.ProtocolError) Then
                resp = New BambooHTTPResponse(DirectCast(e.Response, HttpWebResponse).StatusCode)
            Else
                resp = New BambooHTTPResponse(0)
            End If
        End Try

        Return resp
    End Function

Public Function buildMultiPart(ByVal params As NameValueCollection, ByVal boundary As String, ByVal contentType As String, ByVal name As String, ByVal fileName As String)
        Dim data = ""

        For Each key In params.AllKeys
            data += "--" + boundary + vbCrLf
            data += "Content-Disposition: form-data; name=""" + key + """"
            data += vbCrLf + vbCrLf
            data += params(key) + vbCrLf
        Next

        data += "--" + boundary + vbCr + vbLf
        data += "Content-Disposition: form-data; name=""" + name + """;" + " filename=""" + fileName + """" + vbCrLf
        data += "Content-Type: " + contentType + vbCrLf
        data += vbCrLf
        'data += fileData + vbCrLf + vbCrLf
        'data += "--" + boundary + "--" + vbCrLf

        Return data
    End Function

    Public Function uploadEmployeeFile(ByVal employeeId As Integer, ByVal fileName As String, ByVal fileLocation As String)
        Dim request As New BambooHTTPRequest()
        request.url = String.Format("{0}/v1/employees/{1}/files/", Me.baseUrl, employeeId)
        request.method = "POST"

        Dim boundary = "----BambooHR-MultiPart-Mime-Boundary----"

        Dim params = New NameValueCollection
        params.Add("category", "13")
        params.Add("fileName", fileName)
        params.Add("share", "no")

        request.contents = buildMultiPart(params, boundary, "application/pdf", "file", fileName)

        Return http.sendRequestMPF(request, fileLocation)
    End Function

The rest of the code needed can be found on their GitHub https://github.com/BambooHR