I am tryng to use OAuth to authenticate rather that the traditional username , password.
I can successfuly authenticate and receive back a Instanceurl and token.

But how can i construct a SOAP call using these ?
I tried to follow this link but there is no example.

https://developer.salesforce.com/blogs/developer-relations/2011/03/oauth-and-the-soap-api.html

I have tried to  make a request as follows:

strng SOAP_API_SOBJECT_ENDPOINT = "/services/Soap/u/51.0/";
string sobject = "Account"
Account acct = new Account { Name = $"Test123  ", Description = $"Test Description 123[" };

          string restendpoint = InstanceUrl + SOAP_API_SOBJECT_ENDPOINT + sobject;
            var json = JsonConvert.SerializeObject(acct);
            var request = new HttpRequestMessage(HttpMethod.Post, restendpoint);
            request.Headers.Add("Authorization", "Bearer " + AuthToken);
            request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            request.Headers.Add("X-PrettyPrint", "1");
            request.Content = new StringContent(json, Encoding.UTF8, "application/json");
            var response = client.SendAsync(request).Result;
            return response.Content.ReadAsStringAsync().Result;

The result is Internal Server Error.

Is this the correct way to use OAuth with Soap API ?