The GDPR does not specify the form and method of such communication. You may send the DPO´s contact details by email to email@example.com or through the data box (datová schránka) qkbaa2n by labelling the message with “DPO notification” (Oznámení pověřence).
The communication shall include:
• Identification details of the notifying controller or processor
• Identification of the DPO (name, surname)
• Contact details of the DPO (e-mail, phone)
This information is to be provided by a controller or a processor, not by the natural person designated as DPO. This apply also to situations where a natural person provides the DPO services for more entities.
The GDPR does not specify the form in which controllers or processors shall publish the DPO´s contact details. The most appropriate way is a disclosure on a website or, as the case may be, complementary on the official notification board as well.