Fill-in "Subject" on Mailto script

This script pops up a mail window and addresses
it to don@abc.com but when I add
+ "?Subject: Attention" it does not fill in
the Subject field.

How can I fill in the Subject field?

Thank you.



<script>

function popupMessage() {
var to = "don@abc.com";

// Build mail message components
var doc = "mailto:" + to +"?Subject: Attention"

// Pop UP email message window

window.location = doc;
}
</script>

Is it not ?subject=whatever ??
You have subject:

yes it is,

mailto="YourEmail@whatever.com?subject=Subject"