When building an app, it is often deployed in different environments (test, dev, prod), and therefore the endpoint addresses are changing. As the ServiceReferences.ClientCon
You can do it during runtime by using the constructor of the WCF client in SL that takes endpoint configuration name and the address. The endpoint configuration name is just "MyService" in your example. The address argument you provide will override the one included in ClientConfig.
One of the ways to calculate the address of your service during runtime from SL is (I don't guarantee it will work in every environment configuration):
Extra Info:
This may look complicated when you have many services, but it all can be nicely refactored and with a help of Unity made pretty easy to use for any service. For example, I use a helper function which registers a service client with and it's call looks like this: ServicesHelper.RegisterService