It is possible to use WinRT API directly in Unity scripts. However there are limitations and requirements for this:
Below is an example for getting advertising using WinRT API directly:
using UnityEngine;
public class WinRTAPI : MonoBehaviour 
{
    void Update() 
    {
        auto adId = GetAdvertisingId();
        // ...
    }
    string GetAdvertisingId() 
    {
        #if ENABLE_WINMD_SUPPORT
            return Windows.System.UserProfile.AdvertisingManager.AdvertisingId;
        #else
            return "";
        #endif
    }
}