URL user activate

In what context? Are you talking about one that gets put into the email? Is so, that value is filterable with wpmem_notify_filter (it’s the “act_link” value).

You need the user ID, which also is in the filter as “user_id”.

I’d probably do something like this:

add_filter( 'wpmem_notify_filter', function( $settings ) {
    // The user id if you want to link directly to the user:
    $user_id = $settings['user_id'];

    // Change the URL:
    $settings['act_link'] = "https://yourlink.com/your/path/user-edit.php?user_id=" . $user_id;

    // Return your changes:
    return $settings;
});