We maintain a whitelist of attribution providers who have been vetted as supporting our network. However, we do allow advertisers to use their own servers or other custom providers, providing we can investigate their functionality first.
Our documentation on install attribution can be found here:
https://unityads.unity3d.com/help/advertising/s2s-install-tracking
If you would like to submit a request for a new domain to be added to our whitelist, please send example tracking URLs to unityads-support@unity3d.com.