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).

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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);
}