No idea, we're not the developers of UddeIM so can't help you in regards to modifying UddeIM pages. It already says "To: USERNAME" above the message.
If you're wanting to modify how the private message field displays on profile you can do so by editing the PM field in CB > Field Management and adjusting its profile value layout as follows.
Code:
<a href="index.php?option=com_uddeim&task=new&recip=[user_id]">Send [username] a Message</a>