Request for the permission of type System.Net.WebPermission failed - in sandbox plugin

Hello,

I am trying to access a licensing server to activate a product from a plugin in CRM 2013 online. The plugin is in sandbox and I get the error :

Request for the permission of type 'System.Net.WebPermission, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.

The code where this error arise is:

licensingClient = new CustomHardwareIdLicensingClient(LicensingServiceUrl,
tmpl,
productKey,
licenseKeyValidationData,
productIdOnLicensingServer);

licensingClient.AcquireLicense();

where LicensingServiceUrl="http://www.softactivate.com/SampleLicensingService/"

I have studied the CRM 2013 SDK example about the WebClientPlugin which make explicit use of System.Net.WebClient class to access an external site from a plugin, it's ok, but in my case  the access to external site is included in the call:

licensingClient.AcquireLicense();

 and the I get the above error.

Please, if you have any solution to my problem.

Regards, 

Radu Antonache