Error 404 Server remote no found

I changed my code but i have an error 400 Bad request 

(I try with api.dropboxapi.com and with api.dropbox.com , which one is the right one ? )
Dim _command As String
_command = "https://api.dropboxapi.com/oauth2/token "
Dim Request As HttpWebRequest
Request = HttpWebRequest.Create(_command)
Request.Method = "POST"

Request.KeepAlive = True
ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 Or SecurityProtocolType.Tls12 Or SecurityProtocolType.Tls11 Or SecurityProtocolType.Tls
Request.ContentType = "application/x-www-form-urlencoded"
Request.UserAgent = "Mozilla/5.0 (Windows NT 6.3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.154 Safari/537.36"
ServicePointManager.SecurityProtocol = CType(48, SecurityProtocolType) Or CType(192, SecurityProtocolType) Or CType(768, SecurityProtocolType) Or CType(3072, SecurityProtocolType)
Request.AllowAutoRedirect = True

Dim postbody As String =  "&grant_type=refresh_token "
postbody += "&refresh_token=" & _refresh_token
postbody += "&client_id=" & _App_key
postbody += "&client_secret=" & _App_secret

Dim payload = Encoding.UTF8.GetBytes(postbody)
Request.ContentLength = payload.Length

Using dataStream = Request.GetRequestStream()
dataStream.Write(payload, 0, payload.Length)
End Using

Try
Using response = CType(Request.GetResponse(), HttpWebResponse)
   Using reader = New IO.StreamReader(response.GetResponseStream())
   _file = reader.ReadToEnd()
   End Using
End Using
Catch ex As WebException
   Using response = CType(ex.Response, HttpWebResponse) 
   Using reader = New IO.StreamReader(response.GetResponseStream())
   _errore = reader.ReadToEnd()
End Using
End Using
Catch ex As Exception
Throw
End Try