You can use the GSTN (Goods and Services Tax Network) API.

The GSTN API provides various endpoints for fetching GST-related information such as taxpayer details, returns, payments, refunds, and more.

• Obtain your GSTN API credentials, which include a username and password. You can apply for API access on the GSTN portal.
• Then create a new .NET project
• Install the GSTN API client library for .NET. You can use the NuGet package manager to install the library.
• Create an instance of the GSTN API client by passing in your API credentials.

  using GSTN.API.Client;
  
  var client = new GstnApiClient("your-username", "your-password");
  

•  Call the appropriate endpoint to fetch the GST information you need. For example, to fetch the details of a taxpayer, you can use the GetTaxPayerDetails() method:

using GSTN.API.Models;

var gstin = "your-gstin";
var taxpayer = client.GetTaxPayerDetails(gstin);

Console.WriteLine($"Taxpayer name: {taxpayer.BusinessLegalName}");