XMPP Candy - Adding extension to message

I'm trying to add a custom extension to a message sent via Candy chat. I'm reading all the doc but I can't find a fast way to add custom extension value to sent messages. This is normal sent message frame

<body rid='566337158' xmlns='http://jabber.org/protocol/httpbind' sid='7453ea2wbs'><message to='XXX' from='XXX/Candy' type='groupchat' id='9' xmlns='jabber:client'><body xmlns='jabber:client'>Test Text</body><x xmlns='jabber:x:event'><composing/></x></message></body>

I'd like to add and see something like this in sent message:

<my-ext xmlns='mynamespace:my-ext' someField='got it'/>