I need to a add a button to the navigation or ribbon area of the CRM 2015 on-premise. This button should open an existing Silverlight web resource. Is that possible ?
EDIT:
This MSDN article mentions that the URL holds a value of a URL or an HTML web resource, does that mean that Silverlight webresource is not possible ?
Url
Specifies a URL or HTML Web Resource for a page to display in the main frame of the application when this subarea is selected.
You need to create a page to host the Silverlight control, as specified here:
To display a Silverlight web resource outside an entity form or chart, create an HTML web resource to be the host page for the Silverlight web resource. Then use the $webresource: directive to open the HTML web resource.
Once it is created, make sure you reference the page web resource in your site map/ribbon.
There is a good tutorial here on how to set that up.