You can see how your window will look, without actually running your script. This is done by selecting REXX Programmer Center's Resources -> Quick test menu command. This will show your window in a special "test mode" that displays the window in a semi-operational "test mode". None of your "event handlers" are in effect, so the controls and menu do nothing when you operate them. But you can indeed operate them, easily check your TAB order, and see how the window and its controls will look.
When you wish to exit this test mode, either click on the window's close box, or if there is no close box, select the Resources -> Quick test menu command again.