Google mail insert into text window chrome-extension

Has anybody heard of a way to insert text into the message text window of Google Mai开发者_如何学编程l (when writing a new email) by using a Chrome extension?

I thought about inserting HTML via the extension into the class or id field of the message window (if there is any).

With JQuery you can do it like this

function addText(myText)
{
    $content=$('#canvas_frame').contents().find('body div table form iframe').contents().find('body');
    $content.html($content.html()+ myText);
}