Ü isn't even a special character or utf-8 - ü is part of ascii. How does this even fail? Is their database a 7-bit database?
That is not true. Type “man ascii” on macOS or Linux to see everything that is part of ascii.
But I wouldn’t bother memorising that and every other possible way that the other person has to press the keys depending on their keyboard layout and operating system. I’d just tell people to put u instead.
Ascii is 7 bits. What people think of as 8-bit ASCII is actually code page 437, the alternate characters added to the PC BIOS in the original IBM PC. Like UTF-8 it uses the most significant bit in a 1 byte ASCII char to determine if it should use a character from ASCII if 0 or the extended 437 characters which includes ü if 1. https://en.wikipedia.org/wiki/Code_page_437
But that’s not really the point. No matter how many keyboard shortcuts the clerk at the DMV memorizes there is always going to be some text that they just cannot reproduce accurately. Whether it’s an accented character from the exotic land of Spain or some real Zalgo, something is going to get lost. No individual human can correctly deal with all possible textual forms.