show autocomplete (scriptaculous)

I'm using scriptaculous to generate in a autocomplete field

var user_name_auto_completer = new Ajax.Autocompleter('user_name', 'user_name_auto_complete', '/users/auto_complete_for_user_name', {})

It's wokring OK. But I want that when the users clicks on a button then it should show everything. I need something like onclick=" user_name_auto_completer.filter(''); user_name_auto_completer.show()"

Thanks

Christian