To integrate your web application with HubSpot CRM, you can follow these steps:

1. Sign up for a HubSpot CRM account: Visit the HubSpot website (https://www.hubspot.com) and sign up for a HubSpot CRM account. This will provide you with the necessary credentials to access the HubSpot API.
2. Obtain API credentials: Once you have a HubSpot CRM account, you’ll need to obtain API credentials. To do this, go to your HubSpot account settings and navigate to the “Integrations” or “API” section. Generate or retrieve the API key or OAuth access token, depending on the authentication method you prefer to use.
3. Choose an integration method: HubSpot offers multiple integration methods, including REST APIs, Webhooks, and HubSpot Forms. Choose the integration method that best fits your requirements and application architecture.
4. Integrate using REST APIs: If you choose to integrate using REST APIs, you can make HTTP requests to the HubSpot API endpoints to create, update, or retrieve data from HubSpot CRM. Here’s an example of how you can use the HubSpot CRM Contacts API to create a new contact:

   using System;
   using System.Net.Http;
   using System.Text;
   using Newtonsoft.Json;

   public class HubSpotIntegration
   {
       private static readonly HttpClient httpClient = new HttpClient();

       public static async Task CreateContact(string apiKey, string firstName, string lastName, string email)
       {
           // Set up the HTTP client and request
           httpClient.DefaultRequestHeaders.Add("Authorization", $"Bearer {apiKey}");
           httpClient.DefaultRequestHeaders.Add("Content-Type", "application/json");

           // Create a new contact object
           var contact = new
           {
               properties = new List<object>
               {
                   new { property = "firstname", value = firstName },
                   new { property = "lastname", value = lastName },
                   new { property = "email", value = email }
               }
           };

           // Serialize the contact object to JSON
           var json = JsonConvert.SerializeObject(contact);
           var content = new StringContent(json, Encoding.UTF8, "application/json");

           // Make a POST request to create the contact
           var response = await httpClient.PostAsync("https://api.hubapi.com/crm/v3/objects/contacts", content);
           if (response.IsSuccessStatusCode)
           {
               Console.WriteLine("Contact created successfully");
           }
           else
           {
               Console.WriteLine("Failed to create contact");
           }
       }
   }

In this code, the CreateContact method sends an HTTP POST request to the HubSpot CRM Contacts API to create a new contact. It requires the HubSpot API key for authentication. You can pass the API key, along with the contact details (first name, last name, email) to the method to create the contact in HubSpot CRM.

You can explore other HubSpot API endpoints and functionalities based on your integration needs, such as managing deals, companies, tasks, etc.

5. Implement the integration in your web application: Depending on your application architecture, you can integrate the HubSpot CRM functionality in relevant parts of your web application, such as registration forms, lead capture forms, or when certain events occur. You’ll need to call the appropriate HubSpot API endpoints based on the integration method you chose.
6. Test and iterate: Once the integration is implemented, thoroughly test the functionality to ensure data is being synced correctly between your web application and HubSpot CRM. Make any necessary adjustments or enhancements based on your specific business requirements.

HubSpot provides detailed API documentation and guides that can help you further explore the available integration options and functionalities. You can refer to the HubSpot API documentation for more information: https://developers.hubspot.com/docs/api/overview