How do I make sure that services are automatically brought online in an Oracle 19c Pluggable Database?

In an Oracle 19c pluggable database, the service_names parameter has no bearing on whether or not the services are brought up automatically at instance or server restart.

If the service was started in the PDB and you then saved as the pluggable database's state, it willl be brought online together with the pluggable database automatically.

First, create and start your desired service from the PDBS:

SQL> alter session set container=sales;

Session altered.

SQL> exec dbms_service.create_service(service_name=>'online_users', network_name=>'online_users');

PL/SQL procedure successfully completed.

SQL> exec dbms_service.start_service('online_users');

PL/SQL procedure successfully completed.

Switch to your root container:

SYS@cdb>SQL>alter session set container=CDB$ROOT;

Session altered.

Then, save the state of your PDB:

SYS@cdb>SQL>alter pluggable database sales save state;

Pluggable database altered.

Or, alternatively, save all current states of your PDBs:

SYS@cdb>SQL> ALTER PLUGGABLE DATABASE ALL SAVE STATE;

Pluggable database altered.

The service_names parameter is actually deprecated in Oracle 19c. The documentation states
To manage your services, Oracle recommends that you instead use the SRVCTL command-line utility, the GDSCTL command-line utility, or the DBMS_SERVICE PL/SQL package.