stiwdio on "comment form"

Hi

I'm trying to input an initial value and clear it when clicked;

here is the code taken from the includes comment-template.php

'author' => '<p class="comment-form-author">' . '<label for="author">' . __( '' ) . ( $req ? ' <span class="required"></span>' : '' ) . '</label> ' .
		            '<input id="author" name="author" type="text" size="24" /></p>',

however if i add - onclick="this.value=''" i get the parse error below;

Parse error: syntax error, unexpected T_CONSTANT_ENCAPSED_STRING, expecting '

Any help gratefully appreciated;