There are sereval ways to do this. A very popular library to handle json is the Newtonsoft.Json. Probably you already have it on your asp.net project but if not, you could add it from nuget.
Considering you have a response object, include the following namespaces and call the static method DeserializeObject<T> from JsonConvert class:
using Newtonsoft.Json;
using RestSharp;
return JsonConvert.DeserializeObject<T>(response.Content);
On the response.Content, you will have the raw result, so just deserialize this string to a json object. The T in the case is the type you need to deserialize.
For example:
var customerDto = JsonConvert.DeserializeObject<CustomerDto>(response.Content);
Update
Recently, Microsoft has added a namespace System.Text.Json which handle json format on the .Net platform. You could use it calling the JsonSerializer.Deserialize<T> static method:
using System.Text.Json;
var customer = JsonSerializer.Deserialize<Customer>(jsonContent);