CI3 - How to refer to PDO subdrivers from database.php config?

I'm trying to use the pdo_sqlsrv_driver.php that is new in CI3. However, I'm unable to figure out how to refer to the driver by name in database.config

If I try this I get "Invalid DB driver":

$db['local_windows_pdo']['dbdriver'] = 'pdo_sqlsrv';

I've looked for documentation but haven't been able to find how to refer to PDO sub drivers.

Thank you!
- Elijah