Hi again,
This can be done. But currently this plugin does not expose those api. You will need to create 2 api calls, 1 to send message and another to get messages. Then you can use those 2 apis to send message from your app and get messages from your app.

Everything in website is already taken care off.

This plugin already have some REST api call, you can see those code to get an idea how you can create your own. See class-fep-rest-api.php file.