Any LSP client should start this server over the commandline with the definition file as it's only argument (e.g. dotnet "path/to/autosupport-lsp-server.dll" "path/to/definitionFile.def")
This repository makes use of an git submodule. Make sure to use the according git commands.
Most notably git clone --recurse-submodules to clone or git submodule update --init if you've already cloned it.