ON ACTION block

Use ON ACTION blocks to execute a sequence of instructions when the user raises a specific action. This is the preferred solution compared to ON KEY blocks, because ON ACTION blocks use abstract names to control user interaction.
ON ACTION zoom 
   CALL zoom_customers() RETURNING st, cust_id, cust_name 
Add the INFIELD field-spec option to make the runtime system enable/disable the action automatically when the focus enters/leaves the specified field:
ON ACTION zoom INFIELD customer_city
   LET rec.customer_city = zoom_city()