Hi,
as a starter, I would look at the code in the "plg_telegram.zip" plugin for BF (in the /extensions/addons/ folder of the main download file).
It is in my opinion the easiest to understand plugin.
Then I would simply remove all telegram references in the code, leaving only the barebone functions and the template file display the tab in the backend (but also w/o the telegram references). Also rename the main classname reflecting the new plugin name (likely you gonna call it "cleverreach").
Then rename the installation xml file and editing it to install as a new joomla plugin for breezingforms (basically the plugin name in the xml tags).
After a test install has been working successfully, you can start trying to log into CleverReach with their PHP library and examples.
After installation of your custom plugin, you'll find the files in the "/plugins/breezingforms_addons/" folder.
So let's say you called it "cleverreach", after installation, your files would be located in "/plugins/breezingforms_addons/cleverreach/".
However, try to login using the CleverReach OAuth API in the __construct() function of the main class. Example from CleverReach:
rest.cleverreach.com/howto/#oauth
Please note that you will need to include their library. For some reason the inclusion is not displayed in the top part of the example.
Then carefully follow the output of the login attempts and if you succeed, we can move on.
Regards,
Markus