Adding a field to an existing field group via nlobjForm

So the release notes state the following for field groups support:
"To add fields to field groups, set the tab parameter in nlobjForm.addField(name, type, label, sourceOrRadio, tab) to the name of the field group. In this case, the tab parameter works as a “container id” for either a field group or an actual tab, depending on where you want to place the field"

But this does not appear to work if the form has existing field groups set up in the form designer.

If you do nlobjForm.addField('custpage_field','text','My Label',null,'Primary Information'), the field still ends up at the bottom of the main section. Also tried using the field group ID '-1' and didn't work either.