title: 'Security header is not valid' using PayPal sandbox in .NET
body: <p>I am using the PayPal sandbox in ASP.Net C# 4.0. I added the following web references:</p>
  
  <pre><code>https://www.sandbox.paypal.com/wsdl/PayPalSvc.wsdl
  https://www.paypalobjects.com/wsdl/PayPalSvc.wsdl 
  </code></pre>
  
  <p>When I run this code:</p>
  
  <pre><code>PayPalAPIHelper.PayPalSandboxWS.SetExpressCheckoutReq req = new PayPalAPIHelper.PayPalSandboxWS.SetExpressCheckoutReq()
          {
              SetExpressCheckoutRequest = new PayPalAPIHelper.PayPalSandboxWS.SetExpressCheckoutRequestType()
              {
                  Version = Version,
                  SetExpressCheckoutRequestDetails = reqDetails
              }
          };
  
          // query PayPal and get token
          PayPalAPIHelper.PayPalSandboxWS.SetExpressCheckoutResponseType resp = BuildPayPalSandboxWebservice().SetExpressCheckout(req);
  </code></pre>
  
  <p>In my resp object, the error message says:</p>
  
  <p>Security header is not valid</p>
  
  <p>I was told to give it correct API credentials. I signed up on developer.paypal.com and i'm assuming the email address and password i used are my valid credentials. How and where do I give it my API credentials? Thanks</p>