I would like to add a mandatory checkbox on one of our event registration page. How can I do that please?
You can definitively do that with a custom field! Custom Field -> Used for: Participants -> choose Field Type: checkbox; make one multiple choice option (Label: 'I agree to the terms and conditions'); or: Label: 'Read, Understood and Accepted'.
Stick this new Custom Field into its own profile (so it can can have its own pre-form and/or post-form 'help' i.e. Terms and Conditions) -> make it required (in the profile) so users can't proceed without agreeing/checking the box.
Finally -> stick that profile into your Event Registration page (Event Config -> in the Online Registration tab).