sync_ property
Sync must be exactly "true": delivery here is synchronous, and anything else answers 503 because the queue plane that would run an async dispatch is owned elsewhere. Over REST it rides as ?sync=true (the URL binds over the body); a by-name call states it in its arguments.
Please note: This property should have been non-nullable! Since the specification file does not include a default value (using the "default:" property), however, the generated source code must fall back to having a nullable type. Consider adding a "default:" property in the specification file to hide this note.
Implementation
String? sync_;