If you deal with your clients' Self Assessment, Corporation Tax or PAYE for employers matters, enter your HM Revenue and Customs (HMRC) agent code(s) in this box.
HMRC allocates an agent code (sometimes known as an agent reference number) when you first set up client authorisations for each of these business areas. This code identifies you as an agent with HMRC for those taxes. It is specific to you as an agent and the HMRC business area and should be used on all client authorisation requests for these taxes - either online or paper forms 64-8.
Follow the link below to read more about agent codes and client authorisations.