UNLIMITED LAN BUT INTERNET NEEDS AUTHENTICATION

Hi,
In the company I am working, we have static ip to each PC and PC have unlimited lan access, print ,file sharing etc but no access to internet. Where we tried to access any webpage it redirect to authentication page to have userid and password for internet access. I want to replicate same with mikrotik, kindly help on this, having ax3

Sounds like you should read up on
a. userman
b. radius server
c. hotspot.

https://help.mikrotik.com/docs/display/ROS/RouterOS