After you have successfully created and tested a web service, you can export it to an OpenAPI definition. OpenAPI (formerly Swagger) is an API description format for REST APIs. Various cloud applications such as Nintex Workflow Cloud or Microsoft Flow support the import of OpenAPI files as custom connectors.
The advantage of this is that such custom connectors are easier to configure than a relatively technical, direct web service call.

Create definitions with the WebService Designer

Create definitions via the Management Site