VRCUrls are a locked down version of strings, they can be read from a specific “VRCUrl Text Box” type object (I don’t know the exact name) or defined as a constant in the world and can then be used in video players and this new string loading feature
You can cast a VRCUrl to a string to read the url for displaying it but you cannot cast a string back into a VRCUrl for what I can only assume to be “security reasons”
This means that in VRChat you can only load URLs that are either built-in to the world or have been inputted by a user (which is a really annoying restriction)