As explained in my blog Deprecation of Silverlight Client with ByDesign 1811 you should only use the Silverlight client with SAP Business ByDesign if it is unavoidable. We strongly recommend to use our HTML5 client instead.
However, if it is unavoidable, you can launch the Silverlight client by specifying the client type parameter of your launch URL. For example, if the URL for your Business ByDesign system is https://myXXXXXXX.sapbydesign.com, then you can construct the URL to the Sliverlight client by adding “/sap/public/byd/runtime?client_type=sl”.
The launch URL for starting the silverlight version should look like this: https://myXXXXXXX.sapbydesign.com/sap/public/byd/runtime?client_type=sl
To construct the URL for single sign on the silverlight client, add –sso to domain and add “/sap/byd/runtime?client_type=sl”.
The launch URL for your SAP Business ByDesign system should then look like this: https://myXXXXXXX–sso.sapbydesign.com/sap/public/byd/runtime?client_type=sl.
But again: Be aware that we plan to fully get rid of the Silverlight client as soon as possible.