By default when you use the Silverlight.js library to load a Silverlight control in a page, it will automatically detect whether Silverlight is installed on the visiting client machine, and if not prompt the user to install it via an install image the Silverlight.js library automatically injects into your page. 

This default installation option is a model we call the "indirect install" option - because when the user clicks the install link it automatically opens up a new window with the Silverlight EULA and download page on Microsoft.com displayed. 

The benefit with the indirect install option is that it works with any site (no custom JavaScript required).  The downside, though, is that your visitors will end up navigating to a separate page to install Silverlight, and the overall end user experience won't feel super integrated within the rest of your site.