To apply for a signatory status, download a respective signing form for EPC provider or association of EPC providers or EPC facilitators & other signatory and send it according to the instructions given in the form to the relevant contact from the List of National Code Administrators.
If there is no National Code Administrator allocated for your country, you can send the filled in form to the European Code Co-Administrators:
eu.esco | Andrei Litiu | firstname.lastname@example.org
EFIEES | Katarzyna Wardal | email@example.com
Procedures for the European Code of Conduct for EPC are described here.