The Congrats field value for a specific user can be updated by the following cURL command:
curl -H "X-Atlassian-Token: nocheck" -H "Accept: application/json" -k -u admin:password http://localhost:6710/rest/congrats/1.0/fields/1/value -X POST -d 'username=targetUserName' -d 'date=1981-04-25'