I dont see a 'domain name' field at all like you see. Here's what mine looks like:   I opened a ticket with Support and they said it's not supported, and that i'll have to upgrade to IOS XE.   Their response: At the moment, DHCP Option 15 (domain name suffix) is not supported on Catalyst Switch (CS) firmware in Meraki-mode. This is a known limitation on CS, and unlike the traditional Meraki MS platforms, CS firmware does not currently provide the ability to configure custom DHCP options, including Option 15. In order to utilize DHCP option 15, the switches would need to be migrated to IOS XE  17.15.4 and later.   ... View more

I've just tried it on C9300X and I still get the 'DHCP option code 15 unsupported' error. C9300Xs themselves are on latest firmware. Happens on both the 'old' and the 'new' versions of the dashboard page for DHCP section. ... View more