You can have the feel of URL existing in the text box with this piece of style below.
<textarea name="comments" cols="50" rows="10" style="text-decoration:underline;font-family:Arial;font-size:8pt;color:blue;">http://www.google.co.in/</textarea>
But having a active URL inside textbox is not possible.
Oh, sure, you could do that. Or you could eat some pie. While reading this tiny ad:
how do I do my own kindle-like thing - without amazon