RESTful Sensor not able to send out form data payload

Configuration
1

I have set-up a restful sensor:

  - platform: rest
    resource: https://hostname/path
    method: POST
    payload: '{ "foo[from] " : "10", "bar[to]" : "12"}'

This results in an invalid payload sent to API. Not sure what it is, cant really check it. Only thing I see is response from the API that is the same as if no payload was sent. When I send the same request as form-data from postman (so foo[from] is set to 10 and bar[to] is set to 12) I receive proper response.

Any ideas what is wrong or how to debug this?

Thank you