Add exisitng button - identify hosting entity

Hi,

Here is the scenario I have:

1.  n-n relationship between the contact and a custom entity

2.  subgrid on the custom entity that allows a user to add contacts to the custom entity record

When the "Add Existing Contact" button is clicked from the custom entity, is there a way to identify the host entity; in this case the custom entity?  I want to write some custom javascript when the contact form loads after a user clicks on "Add existing contact" or "Add existing contact -> new" buttons.  I want this custom javascript to execute only when the host entity is my custom entity.  I don't want it to execute anywhere else where a n-n  relationship may exist between the contact entity and another entity.

Any help will be appreciated.