public partial class Form1 : Form
    {
        [DllImport("user32.dll")]
        public static extern void keybd_event(byte bVk, byte bScan, uint dwFlags, IntPtr dwExtraInfo);

        private bool _start;

        public Form1()
        {
            InitializeComponent(); 
        }
      
        private void button1_Click(object sender, EventArgs e)
        {
            if (button1.Text == "Вкл") 
            {
                button1.Text = "Выкл";
                button1.BackColor = Color.Maroon;
                return;
            }

            if (button1.Text == "Выкл")
            {
                button1.Text = "Вкл";
                button1.BackColor = Color.Green;
                Start();
                return;
            }
        }

        private void Start()
        {
            
            while (button1.Text == "Вкл")
            {
              _start = true;
                while (_start)
                {
                    SendKeys.Send("SHIFT");
                    Thread.Sleep(int.Parse(textBox1.Text) * 1000);
                    Application.DoEvents();
                }

                Thread.Sleep(int.Parse(textBox2.Text) * 1000);
            }
        }