add_rewrite_rule… How to use it ?

Hi!
I m trying to use add_rewrite_rule, but it doesn’t work. I tested many code, no success.
So I have installed a new version of wordpress in local to test properly. Same result, no success.
So I have installed a new version of wordpress in local to test properly. Same result, no succes. I don’t know what is the problem. I added this code in function.php of the theme:

function welcome_rewrite_rule()
{
    add_rewrite_rule('^ok$', 'index.php?pagename=test', 'top');
}
add_action('init', 'welcome_rewrite_rule');

When I go to this url :
https://localhost:8888/rew/index.php?pagename=test,
it’s rewritten like this :
https://localhost:8888/rew/?pagename=test
but it would be :
https://localhost:8888/rew/ok

I installed the plugin Rewrite rules inspector, and I can see my rule is registered. But it seems that WordPress has a rule over mine :
(.?.+?)(/[0-9]+)?/?$ index.php?pagename=$matches[1]&page=$matches[2]

I don’t understand how to work with add_rewrite_rule.
Please help !
Thank you.