why complicate your life?
/ipv6 dhcp-server option> add code=23 name=dnstest value="'fe80::ceff:e0ff:fabc:abcd'"
/ipv6 dhcp-server option> print
# NAME CODE VALUE RAW-VALUE
[...]
4 dnstest 23 'fe80::ceff:e0ff:fabc:abcd' fe80000000000000ceffe0fffabcabcd