ShowMandatory Property set during runtime?

Hi all,

I stumbled over this video:
Image may be NSFW.
Clik here to view.

In this video you can see, that the ShowMandatory-property is set during runtime.
My question is, how?

I'm fairly new to Events in NAV but as far as I saw, you are not able to modify properties of a page in a Subscriber-function.
Then, the solution is to export the object, modify the code and import again in background?

Cheers,
Hannes