$ cat udf.sql
CREATE TEMP FUNCTION foo(v STRING) AS (
  (SELECT concat(v, 'foo'))
);

$ cat bq_test.sql
SELECT os, foo(os) FROM telemetry.clients_daily_v6 WHERE submission_date_s3 = '2019-02-10' LIMIT 3

$ cat udf.sql bq_test.sql | bq query
Waiting on bqjob_r167fb1853f68d6fd_000001694a7a7543_1 ... (0s) Current status: DONE
+------------+---------------+
|     os     |      f0_      |
+------------+---------------+
| Darwin     | Darwinfoo     |
| Windows_95 | Windows_95foo |
| Windows_NT | Windows_NTfoo |
+------------+---------------+