I'm not a typst user but from a quick look at the language you should be able to type C\#

You can also write a macro

#let csharp = [C\#]

And then use it as #csharp

Or even `#show "csharp": [C\#]` and then just write `csharp`. Though then you couldn't include the actual text "csharp" in your document, should you wish to, so perhaps defining the variable is a better option.