Done: You can now add parameters such as searchfilter=insource:foo to the search inputbox - it will append that value to the user's search query. Full documentation at https://www.mediawiki.org/wiki/Special:MyLanguage/Extension:InputBox
Please add cirrus specific parameter to preload the search - inputbox
<inputbox>
type=search
namespaces=Wikipedia
deepcat=Wikipedia:Förderung
insource=Raimund
...(all availible cirrus parametes)
</inputbox>
additional: in some cases the "hidden=" -Parameter for Cirrus-enhanced inputbox could be usefull