Today I was requested to make exactly this on this very same version, so here is my code in case someone else needs it:

Create a new javascript file with the next contents:

odoo.define('message_confirm/static/js/composer.js', function (require) {

'use strict';

const { patch } = require('web.utils');

const components = {

​Composer: require('mail/static/src/components/composer/composer.js'),

};

patch(components.Composer, 'message_confirm/static/js/composer.js', {

​ //--------------------------------------------------------------------------

​ // Private

​ //--------------------------------------------------------------------------

​

​ /**

​ * @override

​ */

​ _postMessage() {

​ ​ if (confirm("Are you sure?") == true){

​ ​ ​ this._super();

​ ​ }

​ },

});

});

*note that you may need to update the path and/or module names

Load the javascript in a view:

For some reasont it wont allow me to put html code in here, so just inherit the "web.assets_backend" view and load your javascript file

Add the view to the manifest:

"data": [

​'views/assets.xml',

],

And that's it. Hope it helps out somebody