Depending on the identification document you have and your choice of pickup location, you can apply from overseas. However, you will not be able to immediately use the service unless you are living in Japan and have an identification document that proves you currently live in Japan.
If you have a passport and can pick up the QR code in person at the Shinjuku office or Narita/Haneda/Kansai/Fukuoka airport pickup counters for face-to-face identification verification, then you are more than welcome to apply from overseas. But please be aware that you will have to receive the QR code to install the eSIM after you arrive in Japan.
If you have the following valid documents (NOT expired), then you will be able to apply and immediately use the service from overseas:
1) Residence Card with your address in Japan registered on it
2) Japanese Driver's License
3) Japanese MyNumber Card
Please do contact us if you have further questions regarding this matter.