trying to use C# with multiple files

Hello
I'm trying to use C# with having additional file for an additinal class.
I get a compilation error ...
here is a link:

https://onecompiler.com/csharp/42rvdq2rs

I get the following error:

HelloWorld.cs(13,4): error CS0246: The type or namespace name NewClass1' could not be found. Are you missing an assembly reference? HelloWorld.cs(14,22): error CS0841: A local variable instance1' cannot be used before it is declared

any clue ?
Thanks