How to add S3 event as trigger in Lambda API setup (with ALB)?

Requests from ALB include X-Amzn-Trace-Id, X-Forwarded-For, X-Forwarded-Port, and X-Forwarded-Proto in the header.
So I thought that the request used for testing should also include X-Amzn-Trace-Id, X-Forwarded-For, X-Forwarded-Port, and X-Forwarded-Proto in the header.
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/lambda-functions.html#receive-event-from-load-balancer

An example of an event would include a header like this.

{
    "requestContext": {
        "elb": {
            "targetGroupArn": "arn:aws:elasticloadbalancing:region:123456789012:targetgroup/my-target-group/6d0ecf831eec9f09"
        }
    },
    "httpMethod": "GET",
    "path": "/",
    "queryStringParameters": {parameters},
    "headers": {
        "accept": "text/html,application/xhtml+xml",
        "accept-language": "en-US,en;q=0.8",
        "content-type": "text/plain",
        "cookie": "cookies",
        "host": "lambda-846800462-us-east-2.elb.amazonaws.com",
        "user-agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6)",
        "x-amzn-trace-id": "Root=1-5bdb40ca-556d8b0c50dc66f0511bf520",
        "x-forwarded-for": "72.21.198.66",
        "x-forwarded-port": "443",
        "x-forwarded-proto": "https"
    },
    "isBase64Encoded": false,
    "body": "request_body"
}

Also, looking at GitHub, I found that the error is generated when both "Headers" and "MultiValueHeaders" are not found in the request.
https://github.com/aws/aws-lambda-dotnet/blob/master/Libraries/src/Amazon.Lambda.AspNetCoreServer/ApplicationLoadBalancerFunction.cs#LL65C16-L66C14

            if (lambdaRequest.Headers == null && lambdaRequest.MultiValueHeaders == null)
            {
                throw new Exception("Unable to determine header mode, single or multi value, because both Headers and MultiValueHeaders are null.");
            }