if you want automatic actins, here are excellent, simple codes for auto registration and auto login -
https://wiki.phpbb.com/Add_users
https://wiki.phpbb.com/User_add
https://blog.phpbb.com/tag/hooks/
for autologin:
https://www.phpbb.com/kb/article/phpbb3-cross-site-sessions-integration/
https://www.phpbb.com/kb/article/phpbb3-sessions-integration/