Attaching image as a thumbnail to parent asset (standard page)

I'm trying to create a trigger so when a user adds an image to a page (using an Asset Builder for both image and new page), the image will be attached to its parent (standard page).


Here are my settings for this trigger:



Trigger (enabled)



Events:

Asset Created



Conditions:

Asset is of type: Image



Has a Child/Parent:

has a 'Parent' asset

of type 'Standard Page'



Actions:

Set Thumbnail

Parameter: Set new Thumbnail

Source: GET Variable Name 'XYZ'



So my question is, how do I link the 'GET Variable Name of XYZ' into my asset builder to make this trigger work?



Any help would be much appreciated.



(v4.2.2)



Just realised this whole other thread on the issue I'm having: http://forums.squizsuite.net/index.php?showtopic=6933&st=0