Apex domains cannot have CNAME records.
Question and response
Question
dig onmicrosoft.com. NS
Answer
onmicrosoft.com. 2 NS ns4.bdm.microsoftonline.com.
onmicrosoft.com. 2 NS ns2.bdm.microsoftonline.com.
onmicrosoft.com. 2 NS ns1.bdm.microsoftonline.com.
onmicrosoft.com. 2 NS ns3.bdm.microsoftonline.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0