When you'd like to submit documents which will certify your negative test results for reduce quarantine period, you need to jump to the Health Monitoring Center for Overseas Entrants' website.
Please follow the steps below.
1. Tap the gear icon on the upper right of the top screesn of MySOS.
2. Tap "Application for Special Circumstances"
Please refer to the HCO manual for details.
URL: https://teachme.jp/111284/manuals/15029594/