Hi Nicholas,
Limited-registration events with instant feedback (yours is an extreme example of the same thing, with a single registrant) is a frequently sought feature.
Unfortunately, there isn't any native support for this. Marketo's lead-centric approach doesn't allow you to set program-level properties (in this case, an incrementing program-level token would do the trick) programmatically without using the REST API. And your few daily API calls are so precious that even though you can update a token in response to a form post, you'd be opening up a Denial of Service vulnerability. It's an absolute no-no, even if tempting.
So even though a {{my.token}} could be used -- once it's set -- to change the options displayed on a form, or whether the form is displayed at all, it's setting the token that's out of reach in the real world.
This means we've had to look in other directions to sort-of accomplish the goal.
- One is by using what I call a Resource Lead. This is a special dummy/placeholder lead that represents the program itself. You can increment a score field on this lead for every registration, and you can trigger... well, something... when the score goes over your registration limit. The question is not setting the value in this case but getting the value in order to action it (the opposite of the program token situation above). The development work to do this gets pretty wacky, but everything is kept within Marketo.
- Another is by logging registration events to a remote database (via webhook on form submission, not directly from the client). Then have the browser peek at the remote database (read-only!) before rendering the form. The development effort here is actually easier than the previous bullet, but it takes more moving parts. Pretty cool when it's done. And yet with the lingering feeling that you should have used a different event registration service to do the whole thing (although in truth, 3rd-party event reg services threaten your API limits as well when they flush data back to Marketo).
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