Rewrite custom wordpress url for posts

Hello,

I want to do something very simple.. but i cant figure it out..
I made separate page for the post comments, using this method and its working fine.
Right now the comments url page looks like that:
sitename.com/postname/?show=comments

I want to rewrite this url to sitename.com/postname/comments

The site working with mod_rewrite..

I tried that:

add_action( 'init', 'ss_permalinks' );
    function ss_permalinks() {
        add_rewrite_rule(
            'bestpro/comments',
            'index.php?p=11&show=comments',
            'top'
    );

It’s worked but i need it to be full dynamic.
In this example it’s working only on post id 11 (p=11) with the name bestpro
How can i made it work dynamically and use something like %postname% ?

I toke this code from different post which someone made.. so maybe this code not good for my purpose…

Thanks for the help!