In order to add the TestFairy plugin to a Neptune project, you need to do the following procedure.

Enable the TestFairy plugin

Add the following to the config.xml file in the Neptune Cockpit under Run / Mobile Client / Device.

<plugin name="com.testfairy.cordova-plugin" source="npm"/>

Initialize the TestFairy SDK:

Add the following code to your init script in the App Designer:

document.addEventListener("deviceready", function() {
    TestFairy.begin("APP TOKEN");
});

Remember to replace APP TOKEN with your own app token as displayed in User Preferences