Specify HTTP response codes in Lambda Custom Runtime

I built an AWS Lambda custom runtime function to be triggered through function URLs for a streaming response. I don't see how I can set the HTTP response code, such as 404 or 500. I know it's possible for most HTTP codes in the supported runtimes, like NodeJS. Is it possible with custom runtimes?

I asked this question on StackOverflow with no luck https://stackoverflow.com/questions/78443309/custom-http-response-codes-in-aws-lambda-custom-runtime-with-function-urls?noredirect=1#comment138293602_78443309