So with the SoapRequest function you can call SelecterenOpleiding, SelecterenOpleidingsOnderdelen.

If you go to https://training.coachview.net/Webservice/Geavanceerd.asmx?WSDL you will see which parameters you can add.

const string webserviceKey = "X";
const string webserviceUrl = "https://training.coachview.net/Webservice/Geavanceerd.asmx";

var parameters = new Dictionary<string, string>();
parameters.Add("intOpleidingId", "0");

var result = SoapRequest("SelecterenOpleiding", parameters)

/// <summary>
        /// Execute a Soap WebService call
        /// <param name="action">Get / post action (SelecterenOpleiding, SelecterenOpleidingsOnderdelen)</param>
        /// <param name="parameters">Dictionary with optional paramaters</param>
        /// </summary>
        private string SoapRequest(string action, Dictionary<string, string> parameters)
        {

            //Create Request
            HttpWebRequest request = CreateWebRequest(action);        
      
            using (Stream stream = request.GetRequestStream())
            {

                //add soap body to stream
                CreateSoapBody(action, parameters).Save(stream);

            }
            using (WebResponse response = request.GetResponse())
            {

                //read response
                using (StreamReader rd = new StreamReader(response.GetResponseStream()))
                {

                    var soapResult = rd.ReadToEnd();
                    var doc = new XmlDocument();

                    doc.LoadXml(soapResult);

                    //get node with actual response
                    var result = doc.GetElementsByTagName($"{action}Result");

                    //create json from the node.
                    if(result != null)
                    {
                        return JsonConvert.SerializeXmlNode(result.Item(0));
                    }

                    return "";
                }
            }
        }

        /// <summary>
        /// Create a body
        /// </summary>
        /// <param name="action">Get / post action</param>
        /// <param name="parameters">Dictionary with optional paramaters</param>
        /// <returns></returns>
        private XmlDocument CreateSoapBody(string action, Dictionary<string, string> parameters)
        {

            var xml = new XmlDocument();

            xml.LoadXml("<?xml version=\"1.0\" encoding=\"utf-8\"?>" +
            "<soap:Envelope xmlns:soap=\"http://schemas.xmlsoap.org/soap/envelope/\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">" +
            "<soap:Body>" + 
                $"<{action} xmlns=\"http://coachview.net/webservices\">" +
                    "<aWebserviceAuthentication>" +
                     $"<Key>{webserviceKey}</Key>" +
                    "</aWebserviceAuthentication>" +
                    $"{ CreateParameters(parameters)}" +
                $"</{action}>" +
            "</soap:Body>" +
            "</soap:Envelope>");

            return xml;

        }

        /// <summary>
        /// create string from parameters
        /// </summary>
        /// <param name="parameters">dictionatry of key(string) value(string).</param>
        /// <returns></returns>
        private string CreateParameters(Dictionary<string, string> parameters)
        {

            var result = "";

            foreach(KeyValuePair<string, string> param in parameters)
            {

                result += $"<{param.Key}>{param.Value}</{param.Key}>";

            }

            return result;

        }

        /// <summary>
        /// Create a soap webrequest to webserviceUrl.
        /// </summary>
        /// <param name="parameters">action.</param>
        /// <returns></returns>
        private HttpWebRequest CreateWebRequest(string action)
        {
            HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(webserviceUrl);
            webRequest.Headers.Add("SOAPAction", $"http://coachview.net/webservices/{action}");
            webRequest.ContentType = "text/xml;charset=\"utf-8\"";
            webRequest.Accept = "text/xml";
            webRequest.Method = "POST";
            return webRequest;
        }