Azure Functions - get parameter value in middleware

I am running a v4 Azure Function in an isolated process. It is triggered by a message coming from a Service Bus queue. I have created a simple middleware and would like to get my hands on the incoming data (a simple string). How can I do that from the middleware itself? It doesn't seem FunctionContext is of use in this case.

public class SimpleMiddleware : IFunctionsWorkerMiddleware
{
    public async Task Invoke(FunctionContext context, FunctionExecutionDelegate next)
    {
        await next(context);
    }
}