How to consume HttpClient from F#?

Here is a function that should do what you're looking for (note that you'll have to wrap the code in an asynchronous computation expression in order to use the let! syntax):

let getAsync (url:string) = 
    async {
        let httpClient = new System.Net.Http.HttpClient()
        let! response = httpClient.GetAsync(url) |> Async.AwaitTask
        response.EnsureSuccessStatusCode () |> ignore
        let! content = response.Content.ReadAsStringAsync() |> Async.AwaitTask
        return content
    }

You can use async:

let readString (url: Uri) = async {
    let httpClient = new HttpClient();
    let! response = httpClient.GetAsync(url) |> Async.AwaitTask
    response.EnsureSuccessStatusCode() |> ignore
    return! response.Content.ReadAsStringAsync() |> Async.AwaitTask
}

You'll want to at least read and be aware of the established patterns in Http.fs if you're doing anything with HttpClient in F#.

[See comments] TL;DR ... but Beware the Share. As noted by @pimbrouwers, it should be noted that you don't necessarily have to then use it though - subsetting and/or evolving your own set of helpers in your context can lead you to a better fitting abstraction (and bring you the benefits of the learning on the way).

To this point: It's considered idiomatic practice in F# to keep rarely used and/or overly specific helpers in a non-central location.