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Hi. I'd love to be able to update a custom field via the API, but it seems this isn't possible without getting an "already subscribed" error.
Basically, I'd like for people to be able to subscribe, and then invite them in the welcome email to tell a little more about themselves so I can segment the list. But I'd like to do it as an optional separate step as to not hurt the conversion rate for the initial subscription. Is this possible?
Yes you'll get an Already subscribed message, however any custom fields that you passed into the subscribe API call will be updated.