When XCO is installed, REST API documentation is available as an HTML reference: http://<host_ip>/docs.
The REST API is specified by OpenAPI and Swagger.
Specific API guides for the XCO services are available on the Extreme Networks website. Select ExtremeCloud Orchestrator here: https://www.extremenetworks.com/support/documentation/product-type/software/. And then select the version of XCO you want to work with.