It's actually quite simple. View my attached Picture.
Here's the code from the pic so you can easily copy and paste. :up:
PHP Code:
<div><label for="cb_savecopy"><input type="checkbox" name="savecopy" value="1" id="cb_savecopy" tabindex="1" <if condition="$postpreview">$checked[savecopy]</if> /><phrase 1="private.php?$session[sessionurl]folderid=-1">$vbphrase[save_copy_in_sent_items_folder]</phrase></label></div>