Try

<a href="mailto:myemail@whatever.com?Subject=This is a subject">Mailto tag</a>

"?Subject="
>Is there an option on the mailto tag to place a subject in the email message?
>
>Here's my mailto tag, is there a subject parameter?
>
>

><a href="mailto:myemail@whatever.com">Mailto tag</a>
>

>
>Thanks,
>
>Jerryt