public partial class TestForm : Form
{
    private readonly IKeyboardInterceptor _interceptor;

    public TestForm()
    {
        InitializeComponent();

        // Everytime a key is press we want to display it in a TextBox
        _interceptor = new KeyboardInterceptor();
        _interceptor.KeyPress += (sender, args) => txt.Text += args.KeyChar;
    }

    // Start and Stop capturing keystroks
    private void BtnClick(object sender, EventArgs e)
    {
        if(!_capturing)
        {
            _interceptor.StartCapturing();
            btn.Text = "Stop";
            btn.BackColor = Color.Red;
        }
        else
        {
            _interceptor.StopCapturing();
            btn.Text = "Start";
            btn.BackColor = Color.Lime;
        }
        _capturing = !_capturing;
    }
}