This can also be called the Company Unique Taxpayer Reference (UTR)
Your company’s Unique Taxpayer Reference (UTR) number is 10-digits long. You can find it in correspondence from HM Revenue and Customs (HMRC) under headings:
It’ll usually be shown as part of a larger reference, straight after your company’s 3-digit tax office number.
Enter the 10-digit UTR into HMRC Online Services as a single number, don’t leave blank spaces.