Is it possible to handle this (Fertig/Done) button somehow? Via keycode or something? I want it to behave like Return (iOS) or Go (Android) button.
You can try enterkeyhint
enterkeyhint
Description A hint to the browser for which enter key to display. Possible values: "enter" , "done" , "go" , "next" , "previous" , "search" , and "send" .
Attribute enterkeyhint
Type "done" | "enter" | "go" | "next" | "previous" | "search" | "send" | undefined