anonymous tracking without anonymous_id

[gingerlime]

The documentation states that (emphasis mine):

We call it an Invite Event when you send an event >>>without an anonymous_id and include a data.recipient key<<. You can use these events to trigger invite emails to people you haven't yet identified.

There's also a table below with the list of possible / required params

However, the track_anonymous method requires an anonymous_id, and the code snippet on the documentation page covers only the case with an anonymous_id:

$customerio = Customerio::Client.new("YOUR SITE ID", "YOUR API SECRET KEY", region: Customerio::Regions::US)

$customerio.track_anonymous(anonymous_id, "help_enquiry", :subject => 'anon-events')

Any suggestions on how to continue using anonymous events to be able to trigger campaigns for email addresses, rather than users on customer.io?