can't get Access Token

I am using VB Code, Here is code:

request = DirectCast(WebRequest.Create("https://platform.devtest.ringcentral.com/restapi/oauth/token HTTP/1.1"), HttpWebRequest)

Dim Credidentials As String = "NF3Unhp_R4yBV2QoMNM0yw" & ":" & "3ONXJwFDS2akvz2CaPZX9AaGAzR8uPTUS1h4z80lGtcA"

Credidentials = Convert.ToBase64String(Encoding.Default.GetBytes(Credidentials))

' Add authentication to request

Dim Userdata As String = "grant_type=password&username=12143477228&extension=101&password=POR2008$"

request.ContentType = "application/x-www-form-urlencoded;charset=UTF-8"

request.Method = "POST"

request.Host = "platform.devtest.ringcentral.com"

request.Headers("Authorization") = "Basic " & Credidentials & "&" & Userdata

request.Accept = "application/json"

ServicePointManager.ServerCertificateValidationCallback = AddressOf AcceptAllCertifications

return error : the remote server returned an error: (404) Not Found

I just want to get access token, then I can continue to do next api call.