I want verify the gst number(which is entered in the textbox) valid or not but I unable to find the API

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Net.Http;

namespace WebApplicationfortestingapi
{
    public partial class WebForm1txtbutton : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void btnSubmit_Click(object sender, EventArgs e)
        {
            HttpClient client = new HttpClient(); //for request and response
            string gstId = txtgstNumber.Text;
             string path = API=" + gstId;
            HttpResponseMessage response = client.GetAsync(path).Result; 
            
            if (response.IsSuccessStatusCode)
            {
                var jsonString = response.Content.ReadAsStringAsync(); 
                string result = jsonString.Result;
                if (result == "True")
                {
                    lblmsg.Text = "Valid GST Number";
                }
                else {
                    lblmsg.Text = "Invalid GST Number";
                }
            }
            else {
                lblmsg.Text = "API Not Hitting";
            }
        }
    }