A:
Using FCK Editor
Highlight the text you want to be the link (blue arrow). In the little buttons at the top of the editor there is one called 'Insert/Edit Link' (red arrow) if you click that a mini popup window appears. In the top dropdown (green arrow) select 'Email' and you will be presented with the option to set an email address, email subject and email body. Click 'OK' when you are finished.
Using TinyMCE
Highlight the text you want to be the link (blue arrow). In the little buttons at the top of the editor there is one called 'Insert Link' (red arrow) if you click that a mini popup window appears. In the top input field (green arrow) put mailto: and then the email address you want to send it to. Click insert when you are finished.
Plain HTML
Use the example below:
<a href="mailto:test@test.com?subject=Test Subject&body=Test Body">test@test.com</a>