AWS Lambda Function Direct Post Payload Always Null

Question

I'm trying to use Lambda Functions (C#) with the Function URL for direct access. In postman I'm sending a basic json body that matches the class properties in my input parameter (PostBody). When I execute the POST request, the values are always null tho. Is the input supposed to be something else besides the expected class?

public string FunctionHandler(PostBody input, ILambdaContext context)
{
    LambdaLogger.Log(JsonSerializer.Serialize(input));
    return "Reached Here";
}