This isn't really a webhook-related question; it applies to any trigger campaign setup, and the place to start is always a person's Activity Log. That's where you model the activities you want to trigger on.
In this case the activities would in theory be New Person, plus Data Value Changes triggers for all the fields you wish to sync.
However, this is not going to be workable in practice. You can't call a webhook on every individual field change because it will overwhelm not only Marketo but most likely your CRM as well.
Instead what you need to do, if indeed a webhook is the only way to relay data, is set up a batch-to-trigger config. You have a batch that runs once daily, filtering on people either Created At or Updated At today. That batch calls Request Campaign against a trigger campaign. The trigger campaign calls the webhook. (Batches cannot call webhooks directly.)
I still would be wary of such an arrangement unless you can be assured (and who can be?) that your volume will be low, say no more than 50,000 leads updated in a given day. I'm every bit the superfan of webhooks but you have to know when not to use them.
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