Don't use the JSON API yet, as it's in beta and has several issues, such as non-standard date formatting and poor error handling. The XML API is a better option, which requires generating your own UUIDs, properly formatting dates, and serializing errors correctly. When interacting with Unleashed's API, some keys may be required or optional, so it's best to do sample POSTs with varying amounts of data to determine what works and what doesn't. Additionally, related sub-elements can be defined using various identifiers, such as code or name instead of GUID.