"Add to Steam" on Linux points the shortcut at the launcher binary itself, which
only starts from upstream's self-contained package. Use the script that started
the launcher instead (it sets DOTNET_ROOT and the working directory) when it
says where it is.

--- a/launcher/windows/LanternLauncher/Views/PlayPage.cs
+++ b/launcher/windows/LanternLauncher/Views/PlayPage.cs
@@ -319,9 +319,12 @@
         private void AddToSteam()
         {
             // Windows: Bloodborne.exe, which starts the game with the launcher's settings and waits for
-            // it. Elsewhere the launcher itself with --play, which does the same.
+            // it. Elsewhere the launcher itself with --play, which does the same: through the script
+            // that starts it (LANTERN_ENGINE_LAUNCHER) when there is one.
             var windows = OperatingSystem.IsWindows();
-            var exe = windows ? Path.Combine(Paths.PortDir, "Bloodborne.exe") : Environment.ProcessPath ?? "";
+            var exe = windows ? Path.Combine(Paths.PortDir, "Bloodborne.exe")
+                : Environment.GetEnvironmentVariable("LANTERN_ENGINE_LAUNCHER") is { Length: > 0 } script ? script
+                : Environment.ProcessPath ?? "";
             if (!File.Exists(exe))
             {
                 Prompt.Tell(L.T("Add to Steam"), L.T("Bloodborne.exe is missing from the Lantern Engine folder: reinstall the package."));