It depends on where you want the subform to actually go.
Suppose you have a button inside a subform and you always want the new subform to go on top of the entire stack of subforms:
this.parent.instanceManager.insertInstance(0);
If you want to add it right above whatever instance you're working on, then this code will work:
this.parent.instanceManager.insertInstance(this.parent.index);
Note: You'll need to add or remove reference points depending on what you're using to trigger the event, of course.