Can you add a bit more information on what sort of error you're seeing?
I just tested it out on R80.10 HF take 24 and this is what I got:
[Expert@MDS:0]# mgmt_cli login user ReadOnlyAdmin domain Domain1 > id.txt
Password:
[Expert@MDS:0]# cat id.txt
sid: "uJEFpZ_ljjZiKHBF1zx71gJD71HrTiZwlKA3NWC42Jk"
url: "https://127.0.0.1:443/web_api"
session-timeout: 600
last-login-was-at:
posix: 1503052289706
iso-8601: "2017-08-18T13:31+0300"
read-only: true
api-server-version: "1.1"
[Expert@MDS:0]# mgmt_cli add host name test1 ip-address 1.1.1.1 -s id.txt
code: "generic_error"
message: "Runtime error: Operation is not allowed in read only mode!"
[Expert@MDS:0]#
HTH
Yonatan