To enter directly in Designer, no. In the PLC, they're just BYTEs, so the PLC does not care. Definitely encode as UTF8 to get it to work more easily vs. UTF16.
In Designer, use the $HH escape sequence in string literals to encode high ascii (e.g. 0x80 or greater)
U+00A9
c2 a9
COPYRIGHT SIGN
"The next letter is copyright symbol $C2$A9"
I just read that UTF8 endian-ness doesn't matter. It's raw bytes, MSBYTE...LSBYTE, so $C2$A9 is correct, which makes sense.