enable netlogin web-based
enable web https (you may have to config ssl cert, if you do, then next line)
configure ssl certificate hash-algorithm sha512
save config
show netlogin #to confirm netlogin auth mode web-based is ENABLED
note that i had to create a new admin account to login. the existing one worked for ssh but not for xml (web-based).
create account admin
#you will be prompted for password.
this was done on an X460G2:
X460G2-24p-G4.42 # show version
Switch : 800606-00-05 1523N-44540 Rev 5.0 BootROM: 1.0.2.1 IMG: 16.1.2.14
PSU-1 : Internal PSU-1 800592-00-07 1522A-45204
PSU-2 :
Image : ExtremeXOS version 16.1.2.14 by release-manager
on Tue Oct 6 19:03:00 EDT 2015
BootROM : 1.0.2.1
Diagnostics : 3.1
good luck.