jsapi.js

function handleEvent(e) {
	console.log(e);  
}

const readyEvent = new CustomEvent('reddit.ready', { 
    detail: {
        name: 'fakeConsumer' 
    }
});

document.addEventListener('reddit', handleEvent, true); 
document.dispatchEvent(readyEvent); 

manifest.json

{
    "manifest_version": 2,
    "name": "jsAPI",
    "author": "toolbox team",
    "short_name": "jsAPI",
    "description": "jsAPI.",
    "version": "1.0.0",
    "applications": {
        "gecko": {
            "strict_min_version": "46.0"
        }
    },
    "permissions": [
        "https://*.reddit.com/",
        "http://*.reddit.com/"
    ],
    "content_scripts": [
        {
            "run_at": "document_end",
            "matches": [
                "http://*.reddit.com/*",
                "https://*.reddit.com/*"
            ],
            "js": [
                "jsapi.js"
            ]
        }
    ]
}