Declarative External Services 400 Error Possibly Because of Array of Text Inputs - How To Define

Question

I have previously created many declarative External Services which work with Flow. Getting

400 UnknownOperationException error

with this. Callout is working fine in Postman which is listed here:

{"HITTypeId": "123456",
     "Active": true,
"Notification":{
    "Destination": "desinationURL"
    "Transport": "SNS",
    "Version": "2014-08-15",
    **"EventTypes":["AssignmentAccepted","HITReviewable"]**
}}

My guess is this error has to do with the EventTypes and declaring an array of text inputs which I've not done in Salesforce before. Salesforce schema is here:

"definitions": {
    "updatenotification": {
        "required": [
            "HITTypeId"
        ],
        "type": "object",
        "description": "Body parameters",
        "properties": {
            "HITTypeId": {
                "description": "HITTypeId",
                "type": "string"
            },
            "Active": {
                "description": "Indicates if notifications for HITType are active",
                "type": "boolean"
            },
            "Notification": {
                "type": "object",
                "properties": {
                    "Destination": {
                        "description": "The destination for notification messages",
                        "type": "string"
                    },
                    "Transport": {
                        "description": "The method Amazon Mechanical Turk uses to send the notification",
                        "type": "string"
                    },
                    "Version": {
                        "description": "The version of the Notification data structure schema to use",
                        "type": "string"
                    },
                    "EventTypes": {
                        "type": "array",
                        "items": {
                            "type": "string"
                        }
                    }
                }
            }
        }
    }
}

In Flow I'm using a text collection variable for these inputs vs an Sobject Collection variable. When I used Sobject Collection for this it assigns a key input pair to the input which isnt' what I want in this case.

Answer 1

Problem was more basic and nothing to do with what I thought. I had an incorrect value for part of the header name. Once that was fixed this works as expected.