Like this:

reference @"..\Lib\SomeAssembly.dll";

using SomeAssemblyNamespace;

ClassInSomeAssemblyNamespace.SomeStaticMethod();

or maybe to remain true to the C# standard, use custom attributes:

[reference(@"..\Lib\SomeAssembly.dll")]
[using("SomeAssemblyNamespace")]

ClassInSomeAssemblyNamespace.SomeStaticMethod();