There are two ways to find this:
Select the "Goto Pearson VUE" link on the Take Exam page at https://certification.opengroup.org/take-exam. The summary page shown will include your Candidate ID in the Personal Information.
Alternatively, if applicable, login to your TOGAF 9 Certification account at https://togaf9-cert.opengroup.org, and select View/Edit my Certification Record. The Candidate ID is at the top of the Certification Information. Please note that your Certification ID (CID) is not a Candidate ID.