How to I add row with my current codes

private void btnadd_Click(object sender, EventArgs e)
{
    for (int i = dataGridView1.RowCount - 1; i <= 0; i++)
    {
        DataGridViewRow row = dataGridView1.Rows[i];
       
        if (Convert.ToBoolean(row.Cells["selectrow"].Value = true))
        {
            Formq pan = new Formq();
            pan.ShowDialog(); 
            string may = Formq.mark1;
           productBindingSource4.AddNew();
            dataGridView2.Rows[dataGridView2.Rows.Count - 1].Cells[1].Value = dataGridView1.Rows[i].Cells[1].Value.ToString();
            dataGridView2.Rows[dataGridView2.Rows.Count - 1].Cells[2].Value = dataGridView1.Rows[i].Cells[2].Value.ToString();
            dataGridView2.Rows[dataGridView2.Rows.Count - 1].Cells[3].Value = dataGridView1.Rows[i].Cells[3].Value.ToString();
            dataGridView2.Rows[dataGridView2.Rows.Count - 1].Cells[4].Value = dataGridView1.Rows[i].Cells[4].Value.ToString();
            dataGridView2.Rows[dataGridView2.Rows.Count-1].Cells["quantity"].Value = may;
           int qu = Int32.Parse(dataGridView2.Rows[dataGridView2.Rows.Count - 1].Cells["quantity"].Value.ToString());
            string pp = dataGridView2.Rows[dataGridView2.Rows.Count - 1].Cells[3].Value.ToString();
            Double rpp = Double.Parse(pp);
            Decimal rrpp = (decimal)rpp;

            Decimal tp = rrpp * qu;
            dataGridView2.Rows[dataGridView2.Rows.Count - 1].Cells["tprice"].Value = tp + ".00";

            Double sum = 0;
            for (int f = 0; f < dataGridView2.Rows.Count; f++)
            {
                sum += double.Parse(dataGridView2.Rows[f].Cells[6].Value.ToString());
            }
            string msun = (sum.ToString());
            libt2.Text = msun;

private void btn_Click(object sender, EventArgs e)
{
    this.Hide();
    salesitems op = new salesitems();
    op.Show();
    int i = 0;
   Button btn = sender as Button;
   string btntext = btn.Text;
   kk.getunpaid(int.Parse(btn.Text));
   op.dataGridView2.DataSource = kk.getunpaid(int.Parse(btn.Text));
    
    foreach (var  dy in kk.getunpaid(int.Parse(btn.Text)))
    {
        op.dataGridView2.Rows[i].Cells["quantity"].Value = dy.quantity.ToString();
        op.dataGridView2.Rows[i].Cells["Tprice"].Value = dy.tprice.ToString();
        i++;
        op.libtabsno.Text = dy.tabel_sno_.ToString();
        op.libuser.Text = dy.user.ToString();
        op.libtno.Text = dy.tabel_no_.ToString();
        op.libt2.Text = dy.total.ToString()

productBindingSource4.AddNew();