Targeting 64 bit architectures on Xamarin Android

Go into your Project settings / Build / Android Build / Advanced and enable arm64-v8a as one of your supported ABIs.

And then do a fresh build (or a new archive if you are using that feature of VS).

Note: Make sure you do this for your Release configuration

As @sushihangover mentioned, you just have to Make Sure that on your "Release" configuration, you need to select the arm64-v8a ABI configuration. Remember to "Rebuild All" before you hit "Archive for Publishing" so that you don't run into issues based on cached data. You can view this article for more details and a video.