I'm trying to get a successful /lrService/presets call. I got myself a token and assembled a request according to https://github.com/AdobeDocs/lightroom-api-docs#presets but I always get a 400 Bad Request response with a completely empty body. Any ideas?

That's my request:

{
    "curlopts": {
        "HTTPHEADER": [
            "authorization: Bearer eyJ4NXUiOiJpbXNfbmExLWtleS0xLmNlciIsImFsZyI6IlJ...GElSq1Pg",
            "Content-Type: application/json",
            "x-api-key: 448...2c0f"
        ]
    },
    "json": {
        "inputs": {
            "source": {
                "href": "/files/input.jpg",
                "storage": "adobe"
            },
            "presets": {
                "href": "/files/preset.xmp",
                "storage": "adobe"
            }
        },
        "outputs": {
            "href": "/files/output.jpg",
            "storage": "adobe",
            "type": "image/jpeg",
            "overwrite": true
        }
    }
}