how to use https://api.zomato.com/v2.1/cashless/validateTransaction in vb.net

Question

I'm trying to use https://api.zomato.com/v2.1/cashless/validateTransaction in vb.net; when I'm using it in postman it works fine and gives response, content-type: multipart/form-data; boundary=. but don't have any idea how to send webclient request with parameters, and content-type: multipart/form-data; boundary=.can any one help me please?

I have tried:

Public Function zomatoText() As String Dim str As String = ""

    str = str + "   { " + vbCrLf
    str = str + "   ""mode"":""formdata"", " + vbCrLf
    str = str + "   ""formdata"": [ " + vbCrLf
    str = str + "   { " + vbCrLf
    str = str + "   ""key"": ""res_id"", " + vbCrLf
    str = str + "   ""value"": ""-1"", " + vbCrLf
    str = str + "   ""type"": ""text"" " + vbCrLf
    str = str + "   }, " + vbCrLf
    str = str + "   { " + vbCrLf
    str = str + "   ""key"": ""auth_code"", " + vbCrLf
    str = str + "   ""value"": ""6291"", " + vbCrLf
    str = str + "   ""type"": ""text"" " + vbCrLf
    str = str + "   }, " + vbCrLf
    str = str + "   { " + vbCrLf
    str = str + "   ""key"": ""order_date"", " + vbCrLf
    str = str + "   ""value"": ""2019-01-05"", " + vbCrLf
    str = str + "   ""type"": ""text"" " + vbCrLf
    str = str + "   }, " + vbCrLf
    str = str + "   { " + vbCrLf
    str = str + "   ""key"": ""paid_amount"", " + vbCrLf
    str = str + "   ""value"": ""2014.50"", " + vbCrLf
    str = str + "   ""type"": ""text"" " + vbCrLf
    str = str + "   }, " + vbCrLf
    str = str + "   { " + vbCrLf
    str = str + "   ""key"": ""debug"", " + vbCrLf
    str = str + "   ""value"": ""1"", " + vbCrLf
    str = str + "   ""type"": ""text"" " + vbCrLf
    str = str + "   }, " + vbCrLf
    str = str + "   { " + vbCrLf
    str = str + "   ""key"": ""partner_id"", " + vbCrLf
    str = str + "   ""value"": ""1"", " + vbCrLf
    str = str + "   ""type"": ""text"" " + vbCrLf
    str = str + "   }, " + vbCrLf
    str = str + "   { " + vbCrLf
    str = str + "   ""key"": ""partner_entity_id"", " + vbCrLf
    str = str + "   ""value"": ""1"", " + vbCrLf
    str = str + "   ""type"": ""text"" " + vbCrLf
    str = str + "   } " + vbCrLf
    str = str + "   ] " + vbCrLf
    str = str + "   } " + vbCrLf
    Return str
End Function
Public Function SendZomato() As String
    Try

        Dim strv As String = zomatoText() 'fnc1() 'fnc() 't2() ' test_2()'fnc()


        Dim url = "https://api.zomato.com/v2.1/cashless/validateTransaction"
        Dim client As New WebClient

        client.Headers.Add("Content-Type", "multipart/form-data; boundary=<calculated when request is sent>")
        client.Headers.Add("X-Zomato-Api-Key", "MyKeyGoesHere")
       
        Dim response As String = client.UploadString(url, strv)
        Return response
    Catch ex As WebException
        MsgBox(ex.Message)
    End Try
End Function

Expecting response from request but it is returning "The remote server returned an error: (403) Forbidden."