we only have cancel on requests - not incidents. For me, the main reason to cancel an incident would be that it is resolved, be it by an analyst, the user, or on its own / following a reboot or something, so we only have resolve in place as a closure method for incidents.
I haven't seen many tickets where the person decided they didn't need something fixed that was not working for them, went on with it continuing to be broken & asked us to cancel...
The process should be similar to requests though.
I assume you already have a Cancel Business object in Incidents, with the cancel collection linked to your incident Business object? If so, when using the right-click / Insert Placeholder, you should see the Cancellation Collection there to select from & be able to expand & select the Details attribute from there.