Hi
The only external authentication methods supported by the ESA as of today, are LDAP and RADIUS.
Two factor authentication is not available and is being tracked under the below enhancement requests.
https://bst.cloudapps.cisco.com/bugsearch/bug/CSCum06574/?reffering_site=dumpcr
https://bst.cloudapps.cisco.com/bugsearch/bug/CSCuu64896/?reffering_site=dumpcr
Thanks
Libin Varghese