Generate OpenAPI definition for a Proxy in API Management

I developed and published an API Proxy in API Portal. The API Proxy works Ok and I am invoking it from an iflow through an Open Connector adapter. Now I need to map values from an incoming message into the values of the body of my API Proxy and I am trying to use de message mapping block for this purpose. Whenever I try to upload a JSON definition created from a request sample I get an error (invalid file).

Considering that the mapping block accepts OpenAPI definitions, does anyone know if it's possible to download from API Portal the definition of an API Proxy in OpenAPI format?

Thanks