C# to get access token

Error  Message :Additional information: The remote server returned an error: (500) Internal Server Error.

i am developing  an application in Asp.net mvc to integrate drobbox .

i am creating my web service to get access token from the dropbox by this following  method 

public string PostRequest(string postUrl, string contentType, string parameter, string access_token)
{
using (WebClient client = new WebClient())
{
client.Headers[HttpRequestHeader.ContentType] = contentType;
if (!string.IsNullOrEmpty(access_token))
client.Headers["Authorization"] = access_token;
string result = client.UploadString(postUrl, parameter);
return result;
}
}

here though  i pass all the parameters i get error .

it does not throw error while checking in Insomnia  client -- getting the accesstoken perfectly .

Help me to fix the issue..

Thanks & Regards,

Naveen

when i run my application i get an error message Additional information: The remote server returned an error: (500) Internal Server Error.