function change_admin_URL_idc() {
    $secret_string = 'Your_Word';
    $request_uri = $_SERVER['REQUEST_URI'];

    if (strpos($request_uri, 'wp-login.php') !== false && strpos($request_uri, $secret_string) === false) {
        wp_redirect(home_url());
        exit;
    }
}
add_action('login_head', 'change_admin_URL_idc');



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To login go to:
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https://.../wp-login.php?mySecretString=Your_Word