Fixed it

Sandbox/meson.build

@@ -1,12 +1,32 @@
+# 1) Grab the active C++ compiler
+cpp = meson.get_compiler('cpp')
+ccid = cpp.get_id()  # e.g. 'msvc', 'gcc', 'clang', 'intel', ...
+
+# 2) Decide your D-flag prefix
+if ccid == 'msvc'
+  dflag = '/D'
+else
+  dflag = '-D'
+endif
+
 # Point to the public headers of FluxEngine
 sandbox_inc = include_directories(
   '../FluxEngine/src/public'
 )
 
 executable(
-  'sandbox',
+  'sandbox_Shared',
   'src/private/SandboxApp.cpp',
-  include_directories: sandbox_inc,
-  dependencies       : flux_dep,
-  install            : true,
+  include_directories: [sandbox_inc, flux_inc],
+  link_with: [ flux_shared ],
+)
+
+executable(
+  'sandbox_Static',
+  'src/private/SandboxApp.cpp',
+  include_directories: [sandbox_inc, flux_inc],
+  link_with: [ flux_static ],
+  cpp_args: [
+    dflag + 'FLUX_STATIC'
+  ],
 )

Sandbox/src/private/SandboxApp.cpp

@@ -1,19 +1,9 @@
-#include <Flux.h>
+#include "Flux.h"
 
-class Sandbox : public Flux::Application
+int main (int argc, char** argv)
 {
-public:
-	Sandbox()
-	{
-		
-	}
-	~Sandbox()
-	{
-		
-	}
-};
-
-Flux::Application* Flux::CreateApplication()
-{
-	return new Sandbox();
+	Flux::FluxEngine engine;
+	engine.Start();
+	
+	return 0;
 }