Manually setting the hostname in the DHCP request, and quotation marks are sent to the DHCP server.
Config
set dhcp-client client-if eth0
set dhcp-client client-if eth0 option hostname value infix-r2s-1
This can be seen in the udhcpc process.
udhcpc -f -p /run/dhcp-eth0.pid -t 10 -T 3 -A 10 -a1000 -S -R -x hostname:"infix-r2s-1" -i eth0 -V Infix v23.11.0-121-g051658d-dirty