How to set request body with AWS CRT HTTP Client?

I am creating HTTP requests with the AWS CRT HTTP Client, but I cannot find a way to set the request body. I am using Java 11 with the SDK version 2.27.9. No matter how I configure this synchronous client, my requests do not have a request body. I have tried to set the request body with the SdkHttpFullRequest builder and the HttpExecuteRequest builder. Using both, I can provide a complete example:

SdkHttpClient httpClient = AwsCrtHttpClient.builder().build();
SdkHttpFullRequest sdkHttpRequest = SdkHttpFullRequest.builder()
        .uri(new URI("http://127.0.0.1:5000/post"))
        .contentStreamProvider(ContentStreamProvider.fromUtf8String("'{\"foo\":\"bar\"}'"))
        .method(SdkHttpMethod.POST)
        .appendHeader("accept", "application/json")
        .appendHeader("Content-Type", "application/json")
        .build();
HttpExecuteRequest request = HttpExecuteRequest.builder()
        .request(sdkHttpRequest)
        .contentStreamProvider(ContentStreamProvider.fromUtf8String("'{\"foo\":\"bar\"}'"))
        .build();
HttpExecuteResponse response = httpClient.prepareRequest(request).call();

I am running a local Python Flask app on port 5000 to view the request data, but the body of the request is always empty despite my efforts to set the request body.