How to send plain text in POST request through API Gateway AWS

I'm newbie in whole AWS universe and I'm trying to make a simple/test API. I'b like to send a plain text in a body of POST request but I can't.

The request - https://976c9euh34.execute-api.us-east-1.amazonaws.com/prod/moves/send

I get response:

{
    "message": "Could not parse request body into json: Unrecognized token \'test\': was expecting \'null\', \'true\', \'false\' or NaN\n at [Source: (byte[])\"test\"; line: 1, column: 9]"
}

if I send POST with "test" body. I've tried put Content-Type: plain/text or plain/html but failed. In the lambda I do not try to work with json. Let's say that now my lambda looks like this:

def lambda_handler(event, context):
    file_path = str(time.time()) + '.txt'
    s3 = boto3.client('s3')
    data = "hello world"
    try:
        s3_response = s3.put_object(Bucket=BUCKET_NAME, Key=file_path, Body=data)
    except Exception as e:
        raise IOError(e)
    return {
        'statusCode': 200,
        'body': {
            'data': data
        }
    }

I see some related parameters in Integration Request like

but they don't affect at all.

How should I handle this issue? Where do I need to look at?

UPD:

The mapping alert:

PS: if I put json body it works fine