Certificates of existence for pension purposes related to Germany are generally confirmed by the Embassy. Currently the booking of an appointment is necessary. You do have to come in person. Please bring your passport with you. No fee is charged for German state pensions (DRV). In 2020 you can send the certificate directly to DRV without an embassy stamp.
For general queries related to German retirement pensions and to claim a pension please refer to the official webseite of Deutsche Rentenversicherung.