How do I catch the event when a field's value changes during Jira create issue?

[Todd Shoenfelt]

Hi,

We have a Groovy script Behavior that runs when the Create screen loads.  It populates a pulldown menu,

def field = getFieldByName("Field Name")

with values queried from an external web service,

field.setFieldOptions(fieldOptions) 

I need to detect when the selected pulldown menu's value changes and execute another external web service query.  Can that be done with the existing Behavior on the Create screen before submitted the ticket?