documentclass{article}

usepackage{polyglossia}
usepackage[useui]{eforms}

begin{document}
    textField[ui{value={Übeltäter}}]{Text}{10cm}{1em}
    comboBox{Combo}{10cm}{1em}{(Ärger)(Übeltäter)(Österreich)}
end{document}

** WARNING ** Failed to convert input string to UTF16...