There was a thread on how to set up username and password on adguard but it is not accessible any more.
Anyway, you can set username and password on adguard. Tested on flint 2 router 4.6.4 firmware.
ssh to your router
cd /etc/AdGuardHome
copy original config.yaml file as a backup like
cp config.yaml config.yaml.org
modify config.yaml file in line staring with users, like
users:
- name: admin
password: some_hash_password
Password can be set in hash using linux command
mkpasswd -m bcrypt -R 10 some_hash_password
Paste hash password in adguard config and restart it
/etc/init.d/adguardhome restart
I tested login with android app and home assistant integration. Both are working and I can log in adguard.