Error in call to API function “users/get_current_account”: Unexpected URL params: “access_token”

MainViewModel:

 public async Task<string> Httpclient(string link,string oauthToken)
        {    

            HttpClient client = new HttpClient();
            client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", oauthToken);

            HttpResponseMessage response = await client.PostAsync(link,new StringContent(""));
            response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
            return await response.Content.ReadAsStringAsync();
        }

Get_account_ViewModel:

public class Get_Current_Account_ViewModel
    {
        MainViewModel mainViewModel = new MainViewModel();
        public async Task<Model.Get_Current_Account.RootObject> get_current_account(string _accessToken)
        {
            var query = await mainViewModel.Httpclient("https://api.dropboxapi.com/2/users/get_current_account?access_token=_accessToken",_accessToken);
            if (query != null)
            {
                var get_data = JsonConvert.DeserializeObject<Model.Get_Current_Account.RootObject>(query);
                return get_data;
            }
            else
                return null;
        }

I tried on two ways:

• the first way: I got a problem is

Error in call to API function "users/get_current_account": Unexpected URL params: "access_token" on Dropbox API

at

var query = await mainViewModel.Httpclient("https://api.dropboxapi.com/2/users/get_current_account?access_token=_accessToken",_accessToken);

• second way: Error in call to API function "users/get_current_account": Bad HTTP "Content-Type" header: "text/plain; charset=utf-8". Expecting one of "application/json", "application/json; charset=utf-8", "text/plain; charset=dropbox-cors-hack". when I remove ?access_token=_accessToken at var query.
• four way:

• HttpResponseMessage response = await client.PostAsync(
      link, new StringContent("", System.Text.Encoding.UTF8, "application/json"));    Error in call to API function \"users/get_current_account\": request body: could not decode input as JSON –

Please everyone solve this problem. I can not fix it. thanks.