For use in your programs you can use one of the routines provided here:
https://www.qb64.org/forum/index.php?topic=1385.msg105704#msg105704If it shall be for german only, then you can use my routine from right that mentioned post. If it shall be more international (Western European) then you may also try the routine from the "Best Answer" post, which is provided by forum member "moises".
However, both are INKEY$ replacements only, so you would need to write your own INPUT function, which uses the mentioned routines to collect user input and will concatenate everything in one string until "Enter" is pressed. This will of course make it more difficult to input numbers directly into a numeric variable, as you need to use the VAL function to get a numeric value from a string.
So, there's no perfect solution, the problems with non-US-keyboards and internatinal chars like german umlauts or any AltGr triggered chars are known, but I wait since almost 8 years and nothing has changed yet or will change in the near future.