Please also refer to the following link.
JPO Guidebook for Overseas Users: https://www.jpo.go.jp/resources/report/sonota-info/document/panhu/design_right_injapan.pdf (JPO website)

Japan is one of the contracting parties to the Hague Agreement. Therefore, you can designate Japan in your international application instead of filing a Japanese national design application. However, you are required to appoint a person residing in Japan as your Japanese representative in order to communicate with the Japan Patent Office (JPO). We would be happy to support you in the prosecution of your application in Japan. Please feel free to contact us.

For detailed information, please refer to the JPO site.
Notes for the Designation of Japan in an International Design Application under the Geneva Act of the Hague Agreement | Japan Patent Office (jpo.go.jp)

No procedure is necessary to enter the national phase in Japan. All international applications that designate Japan will be examined by the JPO after international publication.

Japan participates in the WIPO Digital Access Service. For details of the service, please see the JPO page.

[Hague] Submission of priority documents using digital access service (DAS) to Japan Patent Office as the designated Contracting Party | Japan Patent Office (jpo.go.jp)

If you would like to take advantage of the novelty grace period, you will need to fill in relevant information in the “Exception to lack of novelty” section at the time of filing the international application.

Alternatively, you may file a written declaration along with necessary proving documents directly to the JPO within 30 days from the date of publication of the international registration.

After an international application designating Japan is published by the International Bureau (IB), the JPO will assign a domestic application number for each design included in a single application.

After examination by the JPO is completed, and if there are no reasons for refusal, the JPO will assign a domestic registration number and issue a design gazette for each design.

If there is a reason for refusal, the JPO will notify the IB of the refusal within 12 months from the date of publication of the international registration. The IB will send the Notification of Refusal to the applicant or the representative for the international application. Please note that such a refusal will not be sent to the Japanese representative. Therefore, once you receive a refusal notification, it is recommended that you promptly contact your Japanese representative.

The due date for responding to the Notification of Refusal can be extended up to 2 months by filing a request before the expiration date of the designated period.

In the event that the examiner’s decision or a trial decision of refusal has become final, or if the application designating Japan has been withdrawn, you may file a request for refund to the JPO.

Please also refer to the following link.
Seven Tips for Hague Design Applications Designating Japan
JPO Guidebook for Overseas Users:
https://www.jpo.go.jp/resources/report/sonota-info/document/panhu/design_right_injapan.pdf (JPO website)