opening confirm box in code behind

if (TextBox1.Text.Equals(""))
       {
           Label1.Text = "Put your User Id";
           Label1.ForeColor = System.Drawing.Color.Red;
           TextBox1.Focus();
       }
       else if (TextBox2.Text.Equals(""))
       {
           "code here"
           Label1.Text = "Put your Password";
           Label1.ForeColor = System.Drawing.Color.Red;
           TextBox2.Focus();
       }