Hi Luca,
In "Monitoring -> Process Control -> Scenario Processes - Inbound section" you should be able to see the incoming message: the incoming message will be implicitly transformed from JSON into XML.
During the transformation "io" will probably the root tag.
Now please check your scenario step inbound channel definitions:
Which "identification method" did you choose?
In case you took "Root Tag" please try it with identifier = "io".
You could also change to Identification Method = "URL Parameter" and fill Identification Parameter with "urlpar(action)" with Identifier = "PlaceOrder"
Best regards
Bastian