How to generate the COOKIEHASH from JavaScript

By default it’s the MD5 hash of the site URL:

    $siteurl = get_site_option( 'siteurl' );
    if ( $siteurl ) {
        define( 'COOKIEHASH', md5( $siteurl ) );
    }

This includes the scheme but not a trailing slash, so in your example you’d need the hash of https://zeth.dk.

You can also override this by defining COOKIEHASH in wp-config.php with some other value.