// implement the configuration interface (for instance in a configuration class in your app project) or use DefaultHubspotClientConfiguration
using Devpro.Hubspot.Client;

public class AppConfiguration : IHubspotClientConfiguration
{
    // explicitely choose where to take the configuration for Hubspot REST API (this is the responibility of the app, not the library)
}

// configure your service provider (for instance in your app Startup class)
using Devpro.Hubspot.Client.DependencyInjection;

var services = new ServiceCollection()
  .AddHubspotClient(Configuration);

using Devpro.Hubspot.Abstractions;

private readonly IContactRepository _contactRepository;

public MyService(IContactRepository contactRepository)
{
    _contactRepository = contactRepository;
}

public async Task GetContacts()
{
    var contacts = await _contactRepository.FindAllAsync();
}