diff --git a/src/extension-support/tw-unsandboxed-extension-runner.js b/src/extension-support/tw-unsandboxed-extension-runner.js
index 6f44d7cb22f..45312e76ac4 100644
--- a/src/extension-support/tw-unsandboxed-extension-runner.js
+++ b/src/extension-support/tw-unsandboxed-extension-runner.js
@@ -131,6 +131,8 @@ const setupUnsandboxedExtensionAPI = vm => new Promise(resolve => {
 
     global.Scratch = Scratch;
     global.ScratchExtensions = createScratchX(Scratch);
+
+    vm.emit('CREATE_UNSANDBOXED_EXTENSION_API', Scratch);
 });
 
 /**
diff --git a/test/unit/tw_unsandboxed_extensions.js b/test/unit/tw_unsandboxed_extensions.js
index 48cf89c12ed..54dc34fcd34 100644
--- a/test/unit/tw_unsandboxed_extensions.js
+++ b/test/unit/tw_unsandboxed_extensions.js
@@ -417,3 +417,13 @@ test('canEmbed', async t => {
     
     t.end();
 });
+
+test('CREATE_UNSANDBOXED_EXTENSION_API', t => {
+    const vm = new VirtualMachine();
+    vm.on('CREATE_UNSANDBOXED_EXTENSION_API', api => {
+        api.extraStuff = 'aaaa';
+    });
+    UnsandboxedExtensionRunner.setupUnsandboxedExtensionAPI(vm);
+    t.equal(global.Scratch.extraStuff, 'aaaa');
+    t.end();
+});