Could add_query_arg() redirect user to external site?

The function add_query_arg() will generate a link with query parameters.

You should also handle the redirect additionally. Something like this will work:

$url = add_query_arg('key', 'value', 'http://externalpaymentsite.com');
wp_redirect($url)
exit;

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)