Configure multiple search suffix domains in Unifi Security Gateway (dhcp option 119 domain search)
I know that this is quite old, however using the text type as a custom DHCP option didn't work for me. I needed to add a custom code 119 of type hex array and convert the search list to hex. I wrote a simple python script to convert the domain list and just adding the output from that script worked like a charm for me.
I'm sure you've figured this out by now, but you can set a "text" type for a custom DHCP option, then just enter a comma separated domain list. Note: it overrides the default provided by the DHCP server, so you need to include that too.