Just wanted to clarify, by 'protect video URL', did you mean Content protection?
With Flash Media Server 4.5, there are 2 types of Content Protection Schemes available :
1. Protected HTTP Dynamic Streaming - this does not require an external license server
2. Adobe Flash Access DRM Protection
You can find more information regarding the various features supported across the various flavours of FMS here : http://www.adobe.com/ap/products/flashmediaserver/helpmechoose.html
Hope this helps.