Unity Ads Advertising Identifier (iOS)

Sorry if this is somewhere super obvious, but during a fairly long search I didn't find anything solid as to the iOS advertising identifier. At UnityAds, there are multiple modes to choose from, as to whether a device will show monetised ads, or test ads - with the last being all devices show real ads, except for exempted devices. The problem I have is, these devices are described under the option: "devices are identified by their plain text advertising IDs".

With a decent look through the documentation, and searches I did not see any way of finding this, nor in the iPhone system "about" settings. Using SystemInfo.deviceUniqueIdentifier I get an interesting string of characters, but I don't know if this is the advertising ID it wants. In addition, when I put this into the exempt device field, it gives no feedback that that is an actual code it can use.

Any ideas would be appreciated.