Hi Emma,
Hope you are well!
Pleas note that the option to add a cancellation link within the registration process is not possible. However, you can add the Cancel Registration widget on the registration confirmation page, through which the registrant can cancel their registration and will be directed to the cancellation form.
To add this widget, please go to your event > registration > Registration process > click on customize infront of 'Confirmation' page > click on Build on the right > Drag and drop the Cancel Registration widget on the canvas. You can also customize the look and feel of the widget > Publish.
PFB the screenshot of the widget on the confirmation page for your reference :

You can also refer to the article mentioned below for your reference :
How do I let people cancel their registration? == LINK : : https://support.cvent.com/s/communityarticle/How-do-I-let-people-cancel-their-registration
I hope this helps. Please let us know if you have any further questions.
------------------------------
Raashi Semwal Khati
Senior Quality Analyst
Cvent
------------------------------