You have the right URL (http://www.herold.at/servlet/at.herold.sp.servlet.SPWPSearchServlet), but because there's only one "argument", you have to do:
http://www.herold.at/servlet/at.herold.sp.servlet.SPWPSearchServlet?searchterm={query}
Note the ?.
You can right-click on the search field and choose Safari's or Chrome's or Firefox's "Inspect Element" to look at the source code and see what the search query parameter is called. It's not super straightforward, however, so I did it for you.
If you create a custom search in Alfred with this URL:
it should do what you want. The {query} at the end is what Alfred inserts the search query into. search_term is the name of the query form input field (i.e. what the website expects the query to be called).
