GetOAuth2Token.cs

public class TokenContainer
{
    public string Access_token { get; set; }
}

var request = new RestRequest("/core/connect/token", Method.POST);
request.AddHeader("Content-Type", "application/x-www-form-urlencoded");
request.AddParameter("grant_type", "client_credentials");
request.AddParameter("client_id", "CLIENT_ID");
request.AddParameter("client_secret", "CLIENT_SECRET");
request.AddParameter("scope", "SCOPE");
var tokenContainer = await client.ExecuteTaskAsync<TokenContainer>(request);
var token = tokenContainer.Data.Access_token;

//...
request.AddHeader("Authorization", "Bearer " + token);