Jomres 9.8 is distributed with some API functionality built into the core, however you will need to install the "API Core" plugin before you can create clients, which you will need to do to give access to the system.
App key management is done via the Account Details menu option. This option is available to any registered user, however a user's state will depend on what Permissions they can give to that client.
When an API client is created, a Client ID and Secret are automatically created for the client. These are what you will need to use when programming apps to talk to the Jomres API.
When you create a Client you must give them certain permissions, for example a simple Guests app would need to be able to read information about the guest, like their bookings information, modify their favourites etc. A more extensive app for property managers would need to be able to Get and Modify booking information.
Notes
https://github.com/Seldaek/monolog
Logging is done by Monolog. By default logging is sent to the /jomres/temp/monolog directory. For security, we recommend you change the location of this file to one that's outside of the public_html directory tree. Better still, see their API to log directly to another service. You can do this by editing the line
$logger->pushHandler(new StreamHandler('../temp/monolog/jomres_api.log', Logger::DEBUG));
in /jomres/api/classes/logging.class.php