The COMPLETER attribute enables autocompletion for the edit field.
COMPLETER
Form fields with COMPLETER attribute provide suggestions while the end-user types text into the field, it can be used in text edit fields such as EDIT and BUTTONEDIT item types.
Normally, the ON CHANGE trigger is fired for text edit fields when leaving the field and if the content was modified. Form fields defined with the COMPLETER attribute will trigger the ON CHANGE control block when the end user modifies the content of the field.
See Enabling autocompletion for more details.
EDIT f1 = FORMONLY.custname, COMPLETER;