Thunderbird Extensions. How do I set message composer body?

I am currently developing an extension which needs to encrypt the body of the email you are writing. I already did the encryption part, but now I need to display a html in the message composer window. I already know how to extract the text with:

var msgComposer = window.gMsgCompose;
var editor = msgComposer.editor; 
var text = editor.outputToString('text/plain', editor.eNone);

I haven’t found a way to set the mail body however…