Place this in e.g. ~/.local/share/etc/jupyter/jupyter_server_config.d/julialsp.json - that should be enough for julia-ls to work. (obviously install LanguageServer first)

jupyter_julia_lsp.json

{
  "LanguageServerManager": {
    "language_servers": {
      "julia-lsp": {
        "version": 2,
        "argv": ["/home/lbo/bin/julia", "-e", "using LanguageServer; runserver()"],
        "languages": ["julia"],
        "mime_types": ["text/julia", "text/x-julia"],
        "display_name": "JuliaLSP"
      }
    }
  }
}