background.js

const sendMessage = (toastMessage) => {
    chrome.tabs.query({currentWindow:true, active:true}, (tabs) => {
        const tab = tabs[0];
        chrome.tabs.sendMessage(tab.id, {
            toastMessage: toastMessage
        });
    });
}
chrome.contextMenus.onClicked.addListener(({menuItemId}) => {
    if (menuItemId === "show-toast") {
        chrome.storage.sync.get(["message"], ({message}) => {
            sendMessage(message);
        });
    }
});
chrome.runtime.onInstalled.addListener(() => {
    chrome.contextMenus.create({
        id: "show-toast",
        title: "Show Toast!",
        contexts: ["all"]
    });
// default message when extension is installed for the first time
    chrome.storage.sync.set({message: "Hello! This is the default greeting!"});
});