How to submit multi-checkbox response to /:id/submission?

I'm stuck on a form submission issue from my Laravel app to Formstack. All my other fields are making it into my submission, but my multi-checkbox fields do not. I'm trying to return it as an array of the selected options, so

Red [ ]

Blue[X]

Green[X]

Yellow[ ]

Returns "field_id": ['Blue', 'Green']

Am I supposed to submit this back in a different shape? Thanks in advance.