Apex domains cannot have CNAME records.
Question and response
Question
dig mxbmum.freshdesk.com. NS
Answer
mxbmum.freshdesk.com. 5 NS ns-938.awsdns-53.net.
mxbmum.freshdesk.com. 5 NS ns-1963.awsdns-53.co.uk.
mxbmum.freshdesk.com. 5 NS ns-283.awsdns-35.com.
mxbmum.freshdesk.com. 5 NS ns-1295.awsdns-33.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0