Chamada.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            var test = new Teste();
            //Result.Text = test.Somar(Int32.Parse(A.Text), Int32.Parse(B.Text)).ToString();
            List<Posto> teste = test.Rest();
            for (int i = 0; i < teste.Count - 1; i++)
            {
                textBox1.AppendText(test.Rest()[i].nome + "\r\n");
            }
         }
    }
}

MainActivity.cs

using System;
using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;
using System.Collections.Generic;

namespace App6
{
    [Activity(Label = "App6", MainLauncher = true, Icon = "@drawable/icon")]
    public class MainActivity : Activity
    {
       

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);

            // Get our button from the layout resource,
            // and attach an event to it
            Button button = FindViewById<Button>(Resource.Id.MyButton);
            TextView testeRest = FindViewById<TextView>(Resource.Id.restTeste);

            button.Click += delegate {
                Teste teste = new Teste();
                List<Posto> test = teste.Rest();
                for (int i = 0; i < test.Count - 1; i++)
                {
                    testeRest.Text += test[i].nome + "\r\n";
                }
            };
        }
    }
}

Teste.cs

using Newtonsoft.Json;
using RestSharp;
using System;
using System.Collections.Generic;

public class Teste {
    RestClient client;

    public Teste() {
       client = new RestClient("http://localhost:3000");

    }

    public List<Posto> Rest() {
        var request = new RestRequest("api/postos?access_token={access}",Method.GET);
        request.AddUrlSegment("access", "ToJVSIOGTOFwQqx45HIDcQmAD9TRiDu1tDs3xZeZOob3T3VQwuPdvWAajDD8gQ3n");
        IRestResponse response = client.Execute(request);
        var content = response.Content;
        return JsonConvert.DeserializeObject<List<Posto>>(content);

    }
}

public class Posto {
    public string nome { get; set; }
    public string cnpj { get; set; }
    public string endereco { get; set; }
    public string telefone { get; set; }
    public int id { get; set; }
    public int usuarioId { get; set; }
    public int cidadeId { get; set; }
}

👍