Need help with custom rewrite rule

Question

I have a CPT called presskit, and I need to add a custom query var named welcome_box. I have this code right now:

function my_new_url_querystring(){
    add_rewrite_tag( '%welcome_box%','([^&]+)' );

    add_rewrite_rule(
        'presskit/(.?.+?)/box/(.+)/?$',
        'index.php?post_type=presskit&pagename=$matches[1]&welcome_box=$matches[2]',
        'top'
    );

}
add_action( 'init', 'my_new_url_querystring' );

When I try to open mysite.com/presskit/presskit-name/box/john-doe, it gets back to mysite.com/presskit/presskit-name/.

If I open mysite.com/presskit/presskit-name/?welcome_box=john-doe, It works and I can get the query var.

What am I doing wrong? I'm not good with regexes, so maybe the erros is there.

Any tips are welcome!

Update

Thanks to the @Milo answer, I could make it work. Here's the working code:

function my_new_url_querystring(){
    add_rewrite_tag( '%welcome_box%','([^&]+)' );

    add_rewrite_rule(
        'presskit/([^/]+)/message-to/(.*)?/?$',
        'index.php?presskit=$matches[1]&welcome_box=$matches[2]',
        'top'
    );

}
add_action( 'init', 'my_new_url_querystring' );

Answer 1

pagename is for the page post type, if you are trying to pass the slug for the presskit post type, remove post_type and pagename and just set presskit=$matches[1] instead.