Open HMI Builder.
Create a new HMI screen that will be your Alarm popup page.
Select the HMI screen for your project navigation Header.
Add animation to this HMI screen so when the alarm status for the desired alarm goes to active, it opens the Alarm popup screen in a floating window that is configured to be modal and not resizable.

In the above example, we suggested the header navigation as it is always open.
If your project does not have a Navigation Header or it might not be open at all times, you can use VB script to achieve your objective.
User project script to open the alarm popup screen from a function.
Trigger the VB script function from an animation that is triggered by the Alarm status value.