Program.fs

open System
open Microsoft.Owin.Hosting

[<EntryPoint>]
let main argv = 

    let baseAddress = "http://localhost:8888"

    use application = WebApp.Start<Startup.Startup>(baseAddress)

    Console.WriteLine("Server running on {0}", baseAddress)
    Console.WriteLine("hit <enter> to stop")
    Console.ReadLine() |> ignore
    0 // return an integer exit code

Startup.fs

module Startup

    open Owin
    open Microsoft.Owin
    open System
    open System.IO
    open System.Threading.Tasks

    let transform (input: string) =
        sprintf "%s transformed" input

    type public Startup() = 

        let handleOwinContext (context:IOwinContext) =

            use writer = new StreamWriter(context.Response.Body)

            match context.Request.Method with
            | "POST" -> 
                use reader = new StreamReader(context.Request.Body)
                writer.Write(transform(reader.ReadToEnd()))
            | _ ->
                context.Response.StatusCode <- 400
                writer.Write("Only POST")

        let owinHandler = fun (context:IOwinContext) (_:Func<Task>) -> 
            handleOwinContext context; 
            Task.FromResult(null) :> Task

        member x.Configuration (app:IAppBuilder) = app.Use(owinHandler) |> ignore

    let private Main () =
        printfn "%s" (transform "some input")