There are several options to authenticate in service Now.

Basic Authentication

Usually you use this on developing in a dev instance of serviceNow

{
  "BaseAddress": "https://dev77132.service-now.com/api",
  "UserName": "admin",
  "Password": "CQ7K8RfgpaMg"
}

AzureAd Authentication

TIP

The library check if scope has and .default or not to define how it will get access tokens
You can use Credentials or Client Secrets for daemon applications

This uses Azure AD to handle authentication mostly used inside companies.

{
  "Instance": "https://login.microsoftonline.com/",
  "TenantId": "Your company Azure Tenant ID",
  "ClientId": "You CLient Id",
  "ClientSecret": "You Client Secret",
  "BaseAddress": "the servicenow API address",
  "Scope": "the servicenow API address"
}

Those are parameters in the ServiceNow constructor

var config = BasicAuthenticationConfig.ReadFromJsonFile("appsettings.json");
var ServiceNow = new ServiceNow(config);