If you are familiar with API v1, you would have used the X-On-Behalf-Of header to impersonate an end-user while authenticating as an agent. Here are examples of how you would accomplish this equivalently in API v2.
Creating tickets as an end-user
Below is an example curl request that creates a new ticket where firstname.lastname@example.org is the requester. Note that you the end-user's email address and the Zendesk account's token for authentication.