User Defined Variables for Events

I want to use as the event "Unit - Unit Enters Region" for a trigger, but the catch is, the region I want to apply it to is a user defined variable, not a region that you can see in the Region Palette on WE.

Is there any way in which I can use my variables for use in Events?

if u convert ur trigger to jass and replace the region in the add event call, you can enter a variable that u define (ie. udg_<variablename>)
gl

put this custom script line into your map.

call TriggerRegisterEnterRegionSimple(gg_trg_*the trigger name here*, udg_YourVariableName)

No custom text needed :

Trigger - Add to (YourTrigger) the event (Unit - A unit enters MyRegion)