Setprop message are activated when a custom property is modified. If you don't explicity modify it in the SetProp or pass the message, you will not modify the custom property. A setprop must be defined outside any message. Example:
setProp myCustomProperty NewValue answer "Watch out! You are modifying myCustomProperty to " & NewValue pass myCustomProperty end myCustomProperty setProp myCustomProperty2 NewValue set myCustomProperty2 of me to NewValue + 2 end myCustomProperty2 on MouseUp set the myCustomProperty of me to 11 set the myCustomProperty2 of me to 12 answer "Cutomproperty is " & the myCustomProperty of me & return & "Cutomproperty2 is " & the myCustomProperty2 of me end MouseUp
Notice that you can use even not existent custom properties, they will be created automatically when set the respective custom property.