Adding rows to a subform of an existing form

Adding rows to a subform of an existing form

In Zoho Creator I created a "Deals" form, which has a subform to an existing form of "Deal Items".
I want to add deal items to an existing Deal record, using deluge.
I know how to do it for a subform of a blank form (as detailed in this documentation), but how can I do it in a subform of an existing form, using deluge?