The intake form's Base Object refers to the primary record created by an intake form. By default, intake forms support Matters, Accounts, and Contacts as base object options. This capability may be extended using the following techniques.
1. Extend the Base Object Picklist Options
Go to Setup | Create | Objects | Intake Forms
Select the custom field named Base Object.
Add a new value to Base Object's picklist. The value must match the label of the object you are intending to add. For example, if you want to add advpm__Participant__c object, the new value would be Participant.