Thank for the answer, Kyle!

I already tried your first suggestion some days ago, but I put a "%20" instead of a "%2C" for the blank within "No preference" into the URL. Now I got this method working.

But about your second suggestion, I did not get your point (and I have the feeling, this could be the better solution): Where within the list editing page I can change the search mode? Do you mean I have to set an advanced Filter? If yes, which one? Or am I blinded?

I think you mean "Searchable fields" and not "Search criteria". If "Searchable fields" is set to something other than "No", the button "Search users" appears. But that's not the point.

The question from the topic starter (and me) is: How to show the user list and the search form together - without the need to click on "Search users".