I have a customer who wants to send transactional emails triggered by their e-commerce platform to Marketing Cloud using the Rest API. In Salesforce Marketing Cloud, they use the Journey Builder API Entry Event. However, they face a challenge where they ideally need to receive the Subscriber Key from Service Cloud before triggering the email. Unfortunately, there are cases where they receive the Subscriber Key very late, causing delays or even failures in the email trigger sent to Marketing Cloud.
I'm looking for advice on the best way to send the transactional email without including the Subscriber Key in the payload from the e-commerce platform. Instead, I want to capture the Subscriber Key in Marketing Cloud through an asynchronous process. I want to avoid creating duplicate records, adding the contact to the All Subscribers list without a Subscriber Key, and also ensure that the tracking data for existing contacts is preserved.
Any suggestions on how to achieve this?