Related Record ID

Related Record ID

I have a subform (Fulfillment Activities) which is related to a parent form (Service Orders). I want to create a functional URL that lets me open a subform view and only display the records related to the parent. For further explanation:

https://creator.zoho.com/cvanputten.contactpoint/pro-services-ams/#View:Fulfillment_Activities?ID=660708000000316925

will retrieve records for this form based on the record ID of the subform record. However I want to substitute ID for whatever the field name is that holds the parent form's ID. It must exist since the parent form displays the related subform records. I just can't figure out what that name would be.