As described by RFC 6068, mailto allows you to specify subject and body, as well as cc fields. For example:
mailto:username@example.com?subject=Subject&body=message%20goes%20here
User doesn’t need to click a link if you force it to be opened with JavaScript
window.location.href = "mailto:user@example.com?subject=Subject&body=message%20goes%20here";
Be aware that there is no single, standard way in which browsers/email clients handle mailto links (e.g. subject and body fields may be discarded without a warning). Also there is a risk that popup and ad blockers, anti-virus software etc. may silently block forced opening of mailto links.