If it’s like for other region-locked features, you’ll need a Japanese Apple account (formerly Apple ID), which likely means a Japanese payment method and/or billing address, and you need to set your iPhone region to Japan. Furthermore, whatever Apple uses for geolocation (it will include mobile cell and wifi metadata) needs to not indicate a location outside of Japan most of the time.

A VPN doesn’t cut it.

And if they don’t match? Let’s say all of those parmeters indicated a different country…