Apex domains cannot have CNAME records.
Question and response
Question
dig mxbau.freshdesk.com. NS
Answer
mxbau.freshdesk.com. 5 NS ns-2006.awsdns-58.co.uk.
mxbau.freshdesk.com. 5 NS ns-320.awsdns-40.com.
mxbau.freshdesk.com. 5 NS ns-817.awsdns-38.net.
mxbau.freshdesk.com. 5 NS ns-1422.awsdns-49.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0