Hashing fields for Facebook Conversion API in C#

I have successfully called Facebook Conversion API in C# using hardcoded sample data. Code below:

 HttpClient httpClient = new HttpClient();
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer",<MyToken>);

            Int64 unitTimeStamp = DateTimeOffset.UtcNow.ToUnixTimeSeconds();
            var fbData= new
            {
                data = new[] {
        new  {
            event_name="Purchase",
            event_time = 1616695650,
            action_source= "email",
            user_data = new
            {
                 em= "7b17fb0bd173f625b58636fb796407c22b3d16fc78302d79f0fd30c2fc2fc068",
                ph= ""
            },
            custom_data = new
            {
                currency= "USD",
                value= "142.52"
            }

        }
            }
            };



            var postResponse = await httpClient.PostAsJsonAsync("https://graph.facebook.com/v10.0/<MyPixelID/events", fbData);

            postResponse.EnsureSuccessStatusCode();

However when using 'real' data, Facebook requires some types to be hashed using SHA-256 (e.g. email):

https://developers.facebook.com/docs/marketing-api/conversions-api/parameters/customer-information-parameters#normalize-and-hash

I cannot work out how to do this in ASP.NET. I've tried using the below function but this didn't seem to work:

 public static string GenerateSHA256(string input)
        {
            var bytes = Encoding.Unicode.GetBytes(input);
            using (var hashEngine = SHA256.Create())
            {
                var hashedBytes = hashEngine.ComputeHash(bytes, 0, bytes.Length);
                var sb = new StringBuilder();
                foreach (var b in hashedBytes)
                {
                    var hex = b.ToString("x2");
                    sb.Append(hex);
                }
                return sb.ToString();
            }
        }

On a side note, how would Facebook read the data if I did hash it using SHA-256 as I understood this to be irreversible?