In ASP.NET Core, you can send data to a third-party service or API using various methods, such as making HTTP requests. Here's an example of how you can send data to a third party using the HttpClient class:

1. Install the System.Net.Http package if it's not already installed in your ASP.NET Core project.

2. In your controller or service class, inject an instance of HttpClient using the dependency injection system. For example, in a controller:
   

private readonly HttpClient _httpClient;

public YourController(HttpClient httpClient)
{
    _httpClient = httpClient;
}

1. Use the PostAsync method of the HttpClient to send data to the third party. Here's an example:

public async Task<IActionResult> YourAction()
{
    // Create a data object to send
    var data = new { Name = "John", Age = 25 };

    // Serialize the data object to JSON
    var json = JsonConvert.SerializeObject(data);

    // Create a StringContent with the JSON data
    var content = new StringContent(json, Encoding.UTF8, "application/json");

    // Send the POST request to the third party API
    var response = await _httpClient.PostAsync("https://api.example.com/endpoint", content);

    // Check if the request was successful
    if (response.IsSuccessStatusCode)
    {
        // Process the response from the third party API
        // ...
        return Ok();
    }
    else
    {
        // Handle the error
        // ...
        return StatusCode((int)response.StatusCode);
    }
}

In this example, we create an anonymous object representing the data we want to send. We then serialize the object to JSON using the Newtonsoft.Json library (JsonConvert.SerializeObject). Next, we create a StringContent object with the JSON data and specify the content type as "application/json". Finally, we make a POST request to the specified URL using PostAsync, passing the content object.

Make sure to replace "https://api.example.com/endpoint" with the actual URL of the third-party API you want to send data to. Also, handle any exceptions that may occur during the HTTP request for proper error handling.

Remember to configure the HttpClient instance in your ASP.NET Core project's Startup.cs file, specifying the base address and any additional configuration needed, such as authentication or custom headers.

Note: This example uses Newtonsoft.Json for JSON serialization. If you're using .NET Core 3.0 or later, you can use the built-in System.Text.Json namespace for JSON serialization instead.