As of today, there is no programmatic way to send one-off program member custom field data via requested campaigns; the only alternatives are to run a one-off bulk import job or to run a separate form submission call. However, for existing organizations that already have set up REST campaign requests, there is no straightforward way to append this data.
Ideally, passing PMCF data could be done by including something similar to the way token names and values are passed. At this time, this could only support passing records one at a time through the request, but that's a reasonable compromise for most uses.
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