The Munchkin API function associateLead is magical and underused. It's key to synchronizing certain 3rd-party data with your Marketo database, and bizarrely enough was the first Marketo feature I ever used (well before understanding anything about campaigns and flows!).
I have a post set to publish re: tracking engagement with non-Marketo emails, and since that post concentrates on the browser side (how to use specialized tokens in link URLs to associate leads, even when emails are sent via other ESPs) I decided to dedicate a post to how you generate those tokens.
The info below is a must-read preface to the main post, but it'll also be useful on its own to some readers.
WHAT'S AN “ASSOCIATOR TOKEN”?
First, I'll tell you what it's not: it's not the Munchkin cookie, it's not the mkt_tok value that's appended to tracked URLs, and it's not a REST API access_token.