put https://sandbox1-api.unit21.com/v1//entities//add-instruments
Associate an entity with an array of instruments.
Specify the instrument_id of the instrument when associating instruments.
Placeholder entities cannot be updated.
If we do not find any instruments with a corresponding instrument_id in our system, we will create a placeholder for it.
Instrument details can then be supplemented through the /instruments/create or /instruments/update endpoints.