{"error": "invalid_grant", "error_description": "refresh token is malformed"}

I am using this in vb.net with restapi 

Dim link1 As String = "https://api.dropbox.com/oauth2/token"

        Dim client = New RestClient(link1)

        client.Timeout = -1

        mToken = ""

        Dim request = New RestRequest(Method.[POST])

        request.AddHeader("Content-Type", "application/x-www-form-urlencoded")

        request.AddParameter("refresh_token", "UF2xaUcddPOfkhAAAAyAAAIT0EEMOQHza-Gemg5atvn")  -- token changed

        request.AddParameter("grant_type", "refresh_token")

        request.AddParameter("client_id", "xxxxxxx")

        request.AddParameter("client_secret", "vvvvvvvv")

        ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12

        Dim response As IRestResponse = client.Execute(request)

        Console.WriteLine(response.Content)

        Dim xmld As String

        xmld = response.Content

        Dim jsonString As String = xmld

        Dim jsonDynamic As Object = JsonConvert.DeserializeObject(jsonString)

        fieldValue = jsonDynamic("access_token").ToString() ' Replace "fieldName" with the actual field name you want to access

ok I could get the resolution. 

The code which i was getting with offline mode i needed to use once for getting refresh token.  using https://api.dropbox.com/oauth2/token

once i got refresh token now i can regenerate access_token multiple times.