title: Azure B2C Custom Policies: Invalid Request, client secret error
body: <p>I have an Azure B2C Custom Policy to sign up. And when I try to get a refresh token I receive this error:</p>
  
  <pre><code>{"error":"invalid_request","error_description":"AADB2C90079: Clients must send a client_secret when redeeming a confidential grant.\r\nCorrelation ID: 776e4226-467b-4648-b7f7-a9c09715fd68\r\nTimestamp: 2017-10-31 21:12:56Z\r\n"}
  </code></pre>
  
  <p>I'm logging in using this url:</p>
  
  <pre><code>https://login.microsoftonline.com/&lt;MyTenant&gt;.onmicrosoft.com/oauth2/authorize?
  p=B2C_1A_signup_signin
  &amp;client_id=&lt;MyB2CAppID&gt;
  &amp;nonce=defaultNonce
  &amp;redirect_uri=http%3A%2F%2Flocalhost%3A4200
  &amp;scope=openid+offline_access
  &amp;response_type=code
  &amp;prompt=login
  </code></pre>
  
  <p>With the code obtained from the previous url I try to get the access token with this url: </p>
  
  <pre><code>https://login.&lt;MyTenant&gt;.com/exob2ctest.onmicrosoft.com/oauth2/v2.0/token?
  p=B2C_1A_signup_signin
  &amp;grant_type=authorization_code
  &amp;client_id=&lt;MyB2CAppID&gt;
  &amp;redirect_uri=urn:ietf:wg:oauth:2.0:oo
  &amp;code=&lt;MyCode&gt;
  &amp;scope=openid%20offline_access
  &amp;client_secret=&lt;MyB2CAppSecret&gt;
  </code></pre>
  
  <p>After that I'm receiving the error.
  I'm using on the client_secret parameter the key generated on the B2C application referenced in client_id.
  When I tried this with buildt-in policies I get the refresh token without problems. Could it be something missing on my custom policies?</p>
  
  <p>Thanks in advance!</p>