code: func (s *CertificatesService) PurchaseLetsencryptCertificateRenewal(accountID, domainIdentifier string, certificateID int64, certificateAttributes LetsencryptCertificateAttributes) (*certificateRenewalResponse, error) {
  	path := versioned(letsencryptCertificatePath(accountID, domainIdentifier, certificateID) + "/renewals")
  	certificateRenewalResponse := &certificateRenewalResponse{}
  
  	resp, err := s.client.post(path, certificateAttributes, certificateRenewalResponse)
  	if err != nil {
  		return nil, err
  	}
  
  	certificateRenewalResponse.HttpResponse = resp
  	return certificateRenewalResponse, nil
  }
docs: // PurchaseLetsencryptCertificateRenewal purchases a Let's Encrypt certificate renewal.
  //
  // See https://developer.dnsimple.com/v2/certificates/#purchaseRenewalLetsencryptCertificate