If you have the intention to develop software that supports electronic message exchange with Customs or related processes, then you need the information from Message Implementation Guidelines (MIGs).
With the help of this form you can register your organization/company for a (free) account on this site, which makes access to that information possible.
You will usually be identified as an 'Information Subscriber' in the 'Developing Software' community, in which you will also find other relevant information in addition to MIGs.
The community also contains forms to request an account for a colleague and a form to upgrade your account to 'Active subscriber'. This is a subscription for companies that actually develop software and who need test facilities (and a test EORI number).
You can confirm your request by answering a few questions.