public partial class Form1 : Form
    {
        //Declare form2 globally
        Form2 seond;

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            seond = new Form2();
            seond.Show();  

        }

        private void button2_Click(object sender, EventArgs e)
        {
            seond.textBox1.Text = "Hiii...from form1";  
        }
    }