C# – How to use HttpClient PostAsync parameters properly

So I am working on writing an extension class for my project using HttpClient since I am moving over from HttpWebRequest.

When doing the POST request, how do I send a normal string as a parameter? No json or anything just a simple string.

And this is what it looks like so far.

static class HttpClientExtension
    {
        static HttpClient client = new HttpClient();
        public static string GetHttpResponse(string URL)
        {
            string fail = "Fail";
            client.BaseAddress = new Uri(URL);
            HttpResponseMessage Response = client.GetAsync(URL).GetAwaiter().GetResult();
            if (Response.IsSuccessStatusCode)
                return Response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
            else
                return fail;
        }

        public static string PostRequest(string URI, string PostParams)
        {
            client.PostAsync(URI, new StringContent(PostParams));
            HttpResponseMessage response = client.GetAsync(URI).GetAwaiter().GetResult();
            string content = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
            return content;
        }
    }

If you look at this like

client.PostAsync(URI, new StringContent(PostParams));

You can see that I just tried creating new StringContent and passing a string into it and the response returned 404 page not found.
How do I properly use Post.Async(); do I send a string or byte array? Because with HttpWebRequest you would do it like this

public static void SetPost(this HttpWebRequest request, string postdata)
        {
            request.Method = "POST";
            byte[] bytes = Encoding.UTF8.GetBytes(postdata);

            using (Stream requestStream = request.GetRequestStream())
                requestStream.Write(bytes, 0, bytes.Length);
        }