You can track a lead anonymously by associating the cookie value of the anonymous lead with a known lead and updating that lead in the Marketo database. When you configure a task to associate a lead, you can use only the update operation.
For example, you want to synchronize information of an anonymous lead with a known lead to track all activities of the anonymous lead. Create the following flat file source that contains the ID of the known lead and the cookie value of the anonymous lead.
The source file is the following .csv file that you use in a task:
When you choose AssociateLead as the target object and the update operation in a task, the REST API updates the known Lead with the cookie value of the anonymous Lead in the Marketo database and you can track the activities of the anonymous Lead.