Adding properties to form values...
From semantic-mediawiki.org
Fragment of a discussion from Help talk:Adding subobjects using the Page Forms extension
Yes, you need two templates, one for the page and one for the subobjects since there are supposed to be several different items in the box. So you will need to use the multi instance form.
Do not add #set in the item template since you are already passing the respective values to the subobject. I prefer setting all values into the subobject via the form, e.g. by additionally adding {{{field|BOX|hidden|default={{PAGENAME}} }}}
but that's probably a matter of habit. Thus I believe it is easier to grasp which values are actually annotated via the subobject.