If you're already signed up as an organisation for other HM Revenue and Customs (HMRC) online services - such as PAYE for Employers or VAT - you can easily add Corporation Tax to your existing account. To do this, log in to HMRC Online Services with your existing HMRC Online Services User ID and password and follow the 'Enrol for service' link next to 'Corporation Tax (CT)'. There are simple on-screen instructions to guide you through the enrolment process.
If you’re not already signed up for other HMRC online services you need to do so before you can send your return online. See FAQ: 'How do I register as an Individual (or Organisation)'?