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POST /v3/participants/member/options
Should validate that the externalId, if provided, is valid and unassigned (in studies that do this) . Also does not allow ID to be changed.
POST /v3/users/self/externalId
Should validate that the externalId, if provided, is valid and unassigned (in studies that do this) . Also does not allow ID to be changed.
DELETE /v3/users
Unassigns the external ID associated with the user
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