from cognite.client.data_classes.hosted_extractors import MappingWrite, CustomMapping
mapping = MappingWrite(external_id="my_mapping", mapping=CustomMapping("some expression"), published=True, input="json")
res = client.hosted_extractors.mappings.create(mapping)

{
  "items": [
    {
      "externalId": "my.known.id",
      "mapping": {
        "expression": "<string>"
      },
      "input": {
        "type": "protobuf",
        "messageName": "<string>",
        "files": [
          {
            "fileName": "<string>"
          }
        ]
      },
      "published": true,
      "createdTime": 1730204346000,
      "lastUpdatedTime": 1730204346000
    }
  ]
}