I do not exactly understand which one now overwrites the other one, but generally you could first concatenate the URL with the BODY and then assign it to the Email.
$url=$this->createUrl($route,$params); //creates url
$body = $_POST['Model']['body'] . $url; // concatenate body with url
$mail->Body = $body; // either this
$mail->MsgHTML($body); // or this (do not know which works for you)