See
this page in the manual for more details on RADIUS.
Basically, in the "/radius" menu, you add a RADIUS server, tell MikroTik to use it for hotspot, and in the hotspot server profile, you check the "use RADIUS" option. That's about it on the MikroTik end... If a user doesn't exist on the local router, MikroTik will lookup the RADIUS servers configured for hotspot, from first to last.
The hard part comes in configuring the RADIUS server. I've never actually configured one on an external device, so I can't really help there... The only one I've used is MikroTik's "User Manager" package, configured on the same router (for the purposes of using a single source of truth for hotspot, PPPoE and Wi-Fi credentials), but only in testing, because there is a critical feature that "User Manager" doesn't support that I need - "WPA2 - EAP" - so I haven't deployed it for real.