<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:web="http://api.channeladvisor.com/webservices/">
   <soapenv:Header>
      <web:APICredentials>
         <web:DeveloperKey>XXX</web:DeveloperKey>
         <web:Password>XXX</web:Password>
      </web:APICredentials>
   </soapenv:Header>
   <soapenv:Body>
      <web:DoesSkuExist>
         <web:accountID>XXX</web:accountID>
         <web:sku></web:sku>
      </web:DoesSkuExist>
   </soapenv:Body>
</soapenv:Envelope>

<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>
      <DoesSkuExistResponse xmlns="http://api.channeladvisor.com/webservices/">
         <DoesSkuExistResult>
            <Status>Success</Status>
            <MessageCode>0</MessageCode>
            <ResultData>true</ResultData>
         </DoesSkuExistResult>
      </DoesSkuExistResponse >
   </soap:Body>
</soap:Envelope>

// Create your credentials
CA_API_InventorySvc.APICredentials cred = new CA_API_InventorySvc.APICredentials();
cred.DeveloperKey = "dev-key-guid";
cred.Password = "pwd";

// Create the Web Service and attach the credentials
CA_API_InventorySvc.InventoryService svc = new CA_API_InventorySvc.InventoryService();
svc.APICredentialsValue = cred;

// Now just call the method
string sku = "001";
CA_API_InventorySvc.APIResultOfBoolean result = svc.DoesSkuExist("acct-guid", sku);