Hello,

There are 2 REST API function specific to Order Guides. They are submit_guide and checkout_guide, You can use these to create new order guide request from third party tool.

Please search on this API reference link and you will get details about its input and output parameters

https://docs.servicenow.com/bundle/london-application-development/page/integrate/inbound-rest/concept/c_ServiceCatalogAPI.html