I think this is what you want:
object tag;
private void Buttons_Click(System.Object sender, System.EventArgs e)
{
isclicked = true;
// Used "Sender" to know which button was clicked ?
Button btn = sender as Button;
tag = btn.Tag;
}
private void button5_Click(object sender, EventArgs e)
{
if (textBox1.Text != "")
{
SqlConnection conn = new SqlConnection(@"server=.\SQLEXPRESS;
Initial Catalog=DUK1;Integrated Security=True;Pooling=False");
command.Connection = conn;
conn.Open();
command.CommandText = "select product_PRIZE from T_Product where PRODUCT_ID = '" + tag != null ? tag : "" + "'";
textBox5.Text = comando.ExecuteScalar().ToString();
}
}