Form:
$message = _t(
	'ProductForm.PRODUCT_ADDED_LINK',
	'The product was added to {openanchor}your cart{closeanchor}.',
	array(
		'openanchor' => "<a href=\"{$cartPage->Link()}\">",
		'closeanchor' => "</a>"
	)
);

$form->sessionMessage(DBField::create_field("HTMLText", $message),'good');

Template:
<p id="{$FormName}_error" class="message $MessageType">$Message</p>