Can I change a custom field on an Incident through the API?

We are already using the API to retrieve all Incidents and other data from ServiceDesk however we are now trying to update incidents in the system via the API. We are looking to populate a custom field on the Incident type with a custom string based on some logic we will perform in our local program. I cannot find how to send the custom field data via the put http request. In the documentation no custom fields are mentioned.

Thanks,
Andreas