You shouldn't need to add a hidden field. The Set Field step can assign data to a field even if it is not present on your layout.
In layout mode, select the portal, not a field in the portal, and try using either OnObjectExit or OnObjectSave to see which trigger works best for you. The first trigger should trip every time you leave a specific portal row, the second should only trip when data in the portal row has been modified.
I tried both triggers and they don't seem to work. I have an external script which contains the set field line and thats it. I've selected the portal to check which trigger works best for me, all the field information is saved but the set field isnt. What am i missing?
Thanks for your help.
I am a fool. I was using the incorrect shadow table in my Set Field. Sorry for wasting your time.