Setting ResponseEventContext from Embedded Data?

Hi, I'm looking at options to return a particular user key with a survey completed event to help identify who has completed the survey. For other workflow reasons, the survey will be generated using anonymous links rather than distribution links, so the regular RecipientID field can't be used.
According to the API documentation, there is a string called ResponseEventContext on surveyengine.completedResponse.{SurveyID}:
ResponseEventContextStringCustom context that can be set via the ResponseEventContext embedded data. (50 character limit)
Is it actually possible to populate this field with embedded data from the survey, and if so does anyone know how to do that? The documentation is a bit of a dead-end and I can't find ResponseEventContext as a field in the survey flow tool.
Any help (or other ideas) would be greatly appreciated!