Getprop message are activated when a custom property is requested, you will not obtain the value of the custom property, but the value after return. A getprop must be defined outside any message. Example:
getProp myCustomProperty return 23 end myCustomProperty on MouseUp set the myCustomProperty of me to 11 answer the myCustomProperty of me #23 will appear end MouseUp
Notice that getprop usually don't change the value of the custom property.
Getprop is very useful when you need to create a dinamyc complex property of an object. For example in a button script you can write:
getprop lenghtofstring put the length of the text of field "myfield" into temp return temp end lenghtofstring on MouseUp answer the lenghtofstring of me end MouseUp
Notice that you can use even not existent custom properties, just the syntax must adhere to the custom properties syntax.