Dynamic Currency Converter using API in .NET Framework C#

DynamicCurrencyConverter.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Net.Http;
using Newtonsoft.Json.Linq;

namespace Currency_Converter
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            boxKanan.DropDownStyle = ComboBoxStyle.DropDownList;
            boxKiri.DropDownStyle = ComboBoxStyle.DropDownList;
        }

        private async void con_button(object sender, EventArgs e)
        {
            var convert = $"{boxKiri.Text}_{boxKanan.Text}";
            var key = [YOUR_API_KEY];
            var url = $"https://free.currconv.com/api/v7/convert?q={convert}&compact=ultra&apiKey={key}";

            using var client = new HttpClient();
            var content = await client.GetStringAsync(url);
            var CurrencyObject = JObject.Parse(content);

            textKanan.Text = (Double.Parse(CurrencyObject[convert].ToString()) * Double.Parse(textKiri.Text)).ToString();
        }
    }
}