The correct format for the array type parameter in Piano would like this:
https://sandbox.piano.io/api/v3/publisher/resource/attach?aid=ABC123&api_token=api-token&bundle_rid=BR0T1Y72&included_rid=RNXF11W&included_rid=RNBGDVR
In the example above, for the parameter included_rid which should be in the array format, you're able to pass two values RNXF11W and RNBGDVR by adding the same parameter twice to the API call with two different values:
included_rid=RNXF11W&included_rid=RNBGDVR
As you can see, you basically divide the parameter into a series of duplicate parameters. This is true for all cases when you have an array type parameter for the respective API endpoint.