F#: Checking whether a URL returns HTTP Error 404

I am new to programming and F# is my first language. I am currently still very unfamiliar with .NET APIs.

I want to write a function that, given a specific URL, will return TRUE if the HTTP status code is 200, and will return FALSE if it is 404.

Here is the code I have written:

open System.Net

let checkIfUrlIsValid (url: string) =
    try
        let webRequest = HttpWebRequest.Create(url)
        let webRequest.Method = "HEAD"
        use webResponse = webRequest.GetResponse()
        webResponse.StatusCode = HttpStatusCode.OK
    with
        :? System.Net.WebException -> false

However, I get the following error message:

The namespace or module 'HttpWebRequest' is not defined

What went wrong? What changes should I make?