Extract data without Json format - Bigquery

Question

I'm performing the following extract query in CSV and it works perfectly:

EXPORT DATA
  OPTIONS ( uri = 'gs://my_bucket/test*.csv',
    format = 'CSV',
    overwrite = TRUE,
    header = TRUE
    field_delimiter = ','
    ) AS (
  SELECT
    collun1,
    collun2,
    collun3
  FROM
  My_table
);

I would like the same query but in Json format

EXPORT DATA
  OPTIONS ( uri = 'gs://my_bucket/test*.json',
    format = 'json',
    overwrite = TRUE,
    header = TRUE
    field_delimiter = ','
    ) AS (
  SELECT
    collun1,
    collun2,
    collun3
  FROM
  My_table
);

however it doesn't work the way I'm writing, I've looked in the bigquery documentation and haven't found the solution