To avoid the need to add a new field in your database as part of your Bayou implementation, all API models come with an external_id field that can be used to save your identifier. The external_id can be used to retrieve, update or delete any object and can be a string of any format.
If the object is created in your environment, just pass this identifier as an argument when you create it using Bayou's API. Objects created within Bayou's environment can be updated using the Bayou identifier to attach your external_id to them.
The use of external IDs is totally optional.