subsite name during signup

Hello,
I have a mu-plugins to make new blog option checked as default, and the sub-domain name is the username in default, and read only.
Could you help me adjust the code and make the sub-domain name field editable (the same as username in default, but editable)?

<?php

add_action( 'wp_head', function() {
    ?>
    <script type="text/javascript">
    jQuery(function($){
        $('#signup_with_blog').click();

        $( '#signup_blog_url' ).attr('readonly', 'readonly');

        $('#signup_username').keyup(function() {
            var val = $(this).val();
            $('#signup_blog_url').val(val);
        });
    });
    </script>
    <?php
} );

Thanks!
YZ