Can’t Add a custom query var in WordPress

I wanted to add a custom query variable for my wordpress website.

I have tried adding it using this:

function themeslug_query_vars( $qvars ) {
    $qvars[] = 'custom_query_var';
    return $qvars;
}
add_filter( 'query_vars', 'themeslug_query_vars' , 10, 1 );

Also tried this one:

function themeslug_query_vars() {
    add_rewrite_tag('%id%', '([0-9])');
}
add_action('init', 'themeslug_query_vars', 10, 0);

I checked if my custom query var is added using this:

global $wp_query;

var_dump($wp_query->query_vars);

But no luck. Could someone help me with this problem? Thanks!