It is possible to prefill the form via URL parameters like https://en.wikipedia.org/wiki/Special:EmailUser?target=Schnark&wpSubject=summary&wpText=text
But &wpCCMe=1 doesn't check the checkbox.
I don't know whether this worked in 1.17, if so then it's related to bug 30909.
Version: 1.18.x
Severity: enhancement