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I'm afraid that you are not explaining clearly what you want to do and how you're trying to do it. You are talking about a non-existent thing ("JSON key") and giving me an out of context snippet I have no idea what it could be. It sounds like you could be doing either of two things, both wrong in their own ways.

If you are trying to use Akeeba Backup Professional for Joomla with a remote backup service, Akeeba UNiTE or Akeeba Remote CLI the Secret Word is what you need to provide.

Moreover, if you are trying to roll your own connector to the Akeeba Backup JSON API you need to read the Akeeba Backup JSON API Reference. You can't just POST to the endpoint URL. As I've written in the intro page of that documentation:

"For a real world implementation in PHP please take a look at Akeeba Remote Control CLI. You can also run it with the --verbose switch to see the communications taking place between the client and the server. We believe this is the best interactive demonstration of the API implementation."

Finally, please do note that it must fulfill a minimum password complexity requirement for the Akeeba Backup JSON API feature to be enabled. If you're using a Secret Word with the value 1234567890 it will be ignored and will always return an error because it's extremely insecure. All of this is documented under "Secret word". This applies no matter if you're using the JSON API with an already existing connector or if you're trying to roll your own.