How to find Assertion key value?

Hi, 

I am referring MSDN article for Management Libraries for .NET to access my Azure AD account.

I followed all steps from article, but when my application execution starts it gives an error

"An unhandled exception of type 'Microsoft.IdentityModel.Clients.ActiveDirectory.AdalServiceException' occurred in Microsoft.IdentityModel.Clients.ActiveDirectory.dll

Additional information: AADSTS90014: The request body must contain the following parameter: 'client_secret or client_assertion'."

c# block where I caught in error-- 

--------------------------------------------------------

 AuthenticationResult result = context.AcquireToken(
                      ConfigurationManager.AppSettings["apiEndpoint"],
                         ConfigurationManager.AppSettings["clientId"],
                        new Uri(ConfigurationManager.AppSettings["redirectUri"]));

---------------------------------------------------------

my context variable as below

-----------------------------------------------------------

var context = new AuthenticationContext(string.Format(
                  ConfigurationManager.AppSettings["login"],
                  ConfigurationManager.AppSettings["tenantId"]));

-------------------------------------------------------------

and when I am trying to add client_secrete key to code 

--------------------------------------------------------------

 AuthenticationResult  result = context.AcquireToken(
                      ConfigurationManager.AppSettings["apiEndpoint"],
                       new ClientCredential("*******", "******"),
                         new UserAssertion("*****", "client_credentials", "*****"));

--------------------------------------------------------------

throws an error

--------------------------------------------

An unhandled exception of type 'Microsoft.IdentityModel.Clients.ActiveDirectory.AdalServiceException' occurred in Microsoft.IdentityModel.Clients.ActiveDirectory.dll

Additional information: AADSTS50027: Invalid JWT token. AADSTS50027: Invalid JWT token. Token format not valid.

---------------------------------------------

please tell me proper solution where I making mistake and also help to find Assertion key value for application which are added on Azure AD portal.

thanks,

Yogesh