Hello, if you want to add certificates to an Azure AD app registration you can use the MS Graph Update application operation or once there's at least 1 certificated added the addKey operation. Please take a look to the permissions and Azure AD roles required (EG: Application administrator). Alternatively you can use the Azure Portal.
Please let me know if you need more help. If the answer was helpful to you, please accept it and, optionally, provide feedback so that other members in the community can benefit from it.