Add the following to the User Info page:

[if:own_info_page]
[endif:own_info_page]
[elseif:own_info_page]
<br />
<br />
<form action="[send_url]" method="post">
<table border="0" cellspacing="1" cellpadding="1" width="100%">
<tr>
<td class="tableHeader" valign="top"><span class="headingFont">To:</span></td>
<td>&nbsp;<select size="1" name="to[]">
<option value="[user_id]">[username]</option>
</select>
</td>
</tr>
<tr>
<td class="tableHeader"><span class="headingFont">Subject:</span></td>
<td>&nbsp;<input type="text" name="subject" value="[display_subject]" /></td>
</tr>
<tr>
<td class="tableHeader" valign="top"><span class="headingFont">Message</span></td>
<td>&nbsp;<textarea name="message" id="message" cols="35" rows="6" tabindex="2">[display_message]</textarea></td>
</tr>
<tr>
<td></td>
<td><input type="submit" name="submitForm" value="Submit" /></td>
</tr>
</table>
</form>
[endelse:own_info_page]