Take a look at this How to manage assemblies in new C# editor? - #3 by eirannejad
for example if I know System.Text.Json.dll
can be found within application path or is already loaded, I can specify:
// r "System.Text.Json"
using System.Text.Json;
Or if this is a dll on a specific location I can
// r "path/to/your/Library.dll"
using Library;