C# API call "https://api.dropboxapi.com/2/files/list_folder" 400 (Bad Request)

I am getting (400) Bad Request from the below call in c#.

string Url = "https://api.dropboxapi.com/2/files/list_folder";
using (WebClient client = new WebClient())
            {
                NameValueCollection values = new NameValueCollection();
                string datastring = "path=";
                client.Headers.Add("Authorization", "Bearer MYTOKEN");
                client.Headers.Add("Content-Type", "application/json");
                string retval = client.UploadString(Url, "POST", datastring);
            }

Not sure why it is giving 404. I searched online without any solution to this problem.

I changed the 

string datastring = "path=";
string datastring = "path=''";

It also not working