Hi Sara,
The below code will trigger the DO_PREPARE_OUTPUT.
<thtmlb:inputField id = "UNAME"
value = "<%= controller->GV_HTM_VALUE %>"
description = "Forward To"
helpId = "ZSEARCH_FAC"
helpOutputFields="UNAME=FACILITYID;"
helpTriggerSubmit = 'X' />
Thanks,
Ritu