asp.net

Above mrthods are right
But to Add a placeholder in aspx where you want to show panel


And in cs page 
Panel pnl = new Panel(); 
TextBox txt=new TextBox(); 
pnl.Controls.Add(txt); 
Plchold.Controls.Add(pnl); 

Add below code on page load


Panel pnl = new Panel(); 
this.form1.Controls.Add(pnl); 
Label lbl= new Label(); //Controls to add in panel  
lbl.Text="Test"; 


pnl.Width = 200; 
pnl.Height = 200; 
pnl.BackColor = System.Drawing.Color.Red; 
pnl.BorderColor = System.Drawing.Color.Black; 
pnl.Controls.Add(lbl); 
pnl.Visible = true; 
pnl.Enabled = true; 

 try it..

Panel pnl = new Panel();
TextBox txt=new TextBox();
pnl.Controls.Add(txt);
Form.Controls.Add(pnl);

Thanks