Thanks for giving the code. From your code I can see that you are creating new RadWindowManager and the control is not added on page. You need to add the control to container control (Panel, Form, PlaceHolder etc) to work.
. . .
newwindow.Behaviors = WindowBehaviors.Close;
newwindow.VisibleOnPageLoad = true;
this.form1.Controls.Add(manager); // Add the dynamically created control on page
Otherwise, you can add the new window to the RadWindowManager which you already have in your page.