If you have a form interface that you want to open during On Startup, and it's available from the Menu Bar , you can call it using its Menu shortcut instead of calling it from On Startup. Post a key in the On Startup method so that the Menu Item is called, and the On Startup method won't pause while the interface is form running.

Take these steps:
1) Setup a Shortcut key to your Menu 1's Menu Item, such as "1"
2) Add this to the bottom of the On Startup method:

MENU BAR(1)
POST KEY(Ascii("1");command key mask)

The Menu item will be called to start your interface and On Startup will be able to finish.