Udm dns/host_record create --set name="monitor.$(ucr get domainname)" --set a="10.255.253.3" --superordinate "$(udm dns/forward_zone list |sed -ne 's/^DN: //p;T;q')"

Solution:

this part:

.$(ucr get domainname)

needs to be removed from the command specified here (the one I used):

otherwise it results in entries like this:

test.domain.tld.domain.tld