New Thunderbird Plugin: Custom Menu in Message Display Toolbar

I'm new to Thunderbird plugin development, I would like to add a new menu to the message display toolbar with some options

In my manifest.json, I have already configured a menu using the following configuration, as outlined in the Thunderbird WebExtension API documentation:

 "message_display_action": {
    "default_title": "__MSG_ctaMailSummarize__",
    "theme_icons": [
        {
            "dark": "images/bot-icon-light-64.webp",
            "light": "images/bot-icon-dark-64.webp",
            "size": 64
        },
        {
            "dark": "images/bot-icon-light-32.webp",
            "light": "images/bot-icon-dark-32.webp",
            "size": 32
        },
        {
            "dark": "images/bot-icon-light-16.webp",
            "light": "images/bot-icon-dark-16.webp",
            "size": 16
        }
    ],
    "type": "menu"
},

This configuration successfully adds a new menu:

However, I have been unable to add new options; the new menu appears empty.

I haven't found any useful documentation, but I assume I need to use the messageDisplay API, anyway it's unclear to me how to connect my previous configuration in the manifest.json with the API.

Any suggestions?

To add items to Thunderbird's menus, you need to use the menus API.

manifest.json

{
    "manifest_version": 2,
    "name": "SuperMenu",
    "description": "No description",
    "version": "0.1",
    "message_display_action": {
        "default_title": "SuperMenu",
        "type": "menu"
    },
    "permissions": ["menus"],
    "background": {
        "scripts": ["background.js"]
    }
}

background.js

main();

async function main() {
    const contexts = [browser.menus.ContextType.MESSAGE_DISPLAY_ACTION_MENU];
    await browser.menus.create({ contexts, title: 'Foo' });
    await browser.menus.create({ contexts, title: 'Bar' });
}

To handle a click event, add an onclick property to the object passed to menus.create() or use browser.menus.onClicked. For more information on the available parameters, please refer to the corresponding documentation.