How do I define an MSBuild variable for the VSCode C# completion server?

I'm building a mod for Unity based game RimWorld. I have the following assembly references in my .csproj file, using a variable so other people can build the project regardless of where RimWorld is installed:

        <Reference Include="Assembly-CSharp">
            <HintPath>$(RimWorldManagedDir)\Assembly-CSharp.dll</HintPath>
        </Reference>
        <Reference Include="UnityEngine.CoreModule">
            <HintPath>$(RimWorldManagedDir)\UnityEngine.CoreModule.dll</HintPath>
        </Reference>

At build time, I can define $(RimWorldManagedDir) on the command line, and it builds just fine:

msbuild Source/HGAPatchset.csproj -p:Configuration=Release -p:RimWorldManagedDir=/home/nicole/.local/share/Steam/steamapps/common/RimWorld/RimWorldLinux_Data/Managed/

But VSCode can't properly feed this into the language server, likely because I can't define this variable anywhere, and as a result I get autocomplete errors and the following message in the output window of the C# extension:

2025-11-22 13:53:16.735 [warning] [solution/open] [LanguageServerProjectLoader] Project /home/nicole/.local/share/Steam/steamapps/common/RimWorld/Mods/HGAPatchset/Source/HGAPatchset.csproj has unresolved dependencies

I'm very certain I can define this variable somewhere, but I can't seem to find any information on it. How do I define this variable somewhere the C# extension (any completion service) can see it?