You can use one of these two objects to display a popup window when an alarm is triggered:
Direct Window: displays a predefined window based on the state of a bit.
Indirect Window: displays a window based on the value in a register.
You can use your alarm to set the state of a bit or load a value into a register which will trigger the window you want to display.
If you want the popup window to be displayed on any screen in the project when the alarm is triggered, place the window object on the Common Window.