How to add externalId to a Timeseries

I created a set of timeseries via the API. The Python examples do not show how to include an externalId nor do they say one is really needed. Looks like they need one, otherwise they don’t work in Charts. That’s all fine. My problem now is to add the externalId to existing timeseries.

I think the right way to do this is with a transform, but I can’t get it right. I can get it to preview correctly, but it fails when I run it.

My transform is:

Select
  name as name,
  concat(name,"123") as externalId
from
  `_cdf`.`timeseries`
    where dataSetId = 8883655420989208;

The error is Request with id 2463250f-1635-9a05-aa9a-709366e80349 to https://westeurope-1.cognitedata.com/api/v1/projects/rok-buandcollaborators-53/timeseries/update failed with status 400: Time series not found. Missing externalIds: [Tank-401-01-TT123].

Looks like it’s trying to query for the timeseries with that externaId when I’m trying to set the externalId. How do I get it to set the value?