hi Complete, so you wish to prefill not only subject but body as well? Not sure why you wish to do this as IMHO it's sender should do this, isn't it?

Anyway here how you need to form mailto line:
<a href="mailto:email@domainexample.net?subject=Subject sample&body=body of your subject">href
mailto example</a>
please note that you don't need to use single quotes as they would get into body of e-mail as well(at least in certain mailers e.g. Thunderbird).
let me know if it helps.