curl --request POST \
  --url https://{cluster}.cognitedata.com/api/v1/projects/{project}/writeback/sap/mappings \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "items": [
    {
      "externalId": "my.known.id",
      "expression": "\n{\n \"SAPFieldA\": substring(input.CDFFieldA,0,40), \n \"SAPFieldB\": input.CDFFieldB, \n}\n"
    }
  ]
}
'

{
  "items": [
    {
      "externalId": "my.known.id",
      "expression": "\n{\n \"SAPFieldA\": substring(input.CDFFieldA,0,40), \n \"SAPFieldB\": input.CDFFieldB, \n}\n",
      "createdTime": 1730204346000,
      "lastUpdatedTime": 1730204346000
    }
  ]
}