Found another way to soulve this, if anyone know the answear on my original question please tell, good to know anyway :)
Simple like this with webshim:
@Html.TextBoxFor(x => x.MessageHeader)
<textarea id="MessageText" name="MessageText" data-dependent-validation='{"from": "MessageHeader", "prop": "required", "toggle": true}' />
More info about this here: http://afarkas.github.io/webshim/demos/demos/webforms/4-webforms-custom-validity.html