Asked to add reference to System.Runtime.dll

You can always hit F4 to bring up the query properties, and if the query you just ran failed for that assembly being missing, it'll offer to add it for you. Once it does, you can click 'Set as default for new queries'.

I've hit it as well a couple of times when I was using certain NuGet packages - I don't get it if I just run an empty expression or something simple like 2+2, so I just figured it was a dependency some package took, but haven't really dug into it much.

Try the latest beta - it makes this much easier. Now when you press F4 to add the reference, it offers the option to always add missing references without any prompting.