Custom GPT Action + Random.org API Key Issue

Dear friends, I could really use some help!

I’m stuck trying to figure out how to handle personal API keys when setting up Actions for my custom GPTs.

Let me give you a specific example to make it clearer:
I’ve got a model that I want to integrate with random.org, which requires the API key to be passed strictly in the body of the request, since their API is based on the JSON-RPC protocol.

Now, when I try to hardcode the key directly into the schema (by defining it as a constant), it doesn’t work — because, judging by the debug logs, any sensitive data like keys is just being mocked or replaced with a placeholder. In my case, all I see is a string of zeros instead of the actual key.

I also tried passing it through the header, but that didn’t work either, since random.org doesn’t follow a typical REST API pattern.

So… what’s the move here?
Is setting up a proxy the only real solution?
Has anyone run into this before?