How to Parse a JSON with C#
Let a JSON be:
  {
  "access_token": "2564231sdfsdf1sd",
  "expires_in": 3600,
  "token_type": "Bearer"
  }
There are at least two ways I know to access to the values with C#:
  - With the Deserialization way
  
- With the JSON PAth Query way
Both of them requires you to install and reference 
https://www.newtonsoft.com/json
The Deserialization way
With this method, you have to define a class that match the JSON and then perform the deserialisation:
  using Newtonsoft.Json;
  using Newtonsoft.Json.Serialization;
  // You probably need more
  namespace SSFS
  {
    public class Token{
      public string access_token {get; set;}
      public int expires_in {get; set;}
      public string token_type {get; set;}
    }
    public class Utils {
      string jsonString = @"{
        "access_token": "2564231sdfsdf1sd",
        "expires_in": 3600,
        "token_type": "Bearer"}";
        
      string getTokenValue(){
        Token j = JsonConvert.DeserializeObject(jsonString);
        return j.access_token;
      }
    }
  }
JSON PAth Query way
With this method, you have to parse the JSON then perform the Query
  using Newtonsoft.Json;
  using Newtonsoft.Json.Serialization;
  using Newtonsoft.Json.Linq;
  // You probably need more
  namespace SSFS
  {
    public class Utils {
      string jsonString = @"{
        "access_token": "2564231sdfsdf1sd",
        "expires_in": 3600,
        "token_type": "Bearer"}";
        
      string getTokenValue(){
        JObject o = JObject.Parse(jsonString);
        string accessToken = (string)o.SelectToken("access_token");    
        return accessToken;
      }
    }
  }