Where to add code to module/plugin.

I’ve been trying to use the prebuilt plugin to create an editor mode but I’m not sure I understand where to place new code. It keeps throwing an error and I can’t declare variables or it won’t let me compile. So I have this class that was mostly built except I added the uint32.

class FP23DEdModeToolkit : public FModeToolkit
{
public:

	FP23DEdModeToolkit();
	
	/** FModeToolkit interface */
	virtual void Init(const TSharedPtr<IToolkitHost>& InitToolkitHost) override;
	
	

	/** IToolkit interface */
	virtual FName GetToolkitFName() const override;
	virtual FText GetBaseToolkitName() const override;
	virtual class FEdMode* GetEditorMode() const override;
	virtual TSharedPtr<class SWidget> GetInlineContent() const override { return ToolkitWidget; }

	uint32 TESTINT;

private:

	TSharedPtr<SWidget> ToolkitWidget;
};

Inside the .cpp I have tried declaring it to see if ti would work like this:

FP23DEdModeToolkit::FP23DEdModeToolkit()
{
}

void FP23DEdModeToolkit::Init(const TSharedPtr<IToolkitHost>& InitToolkitHost)
{
	struct Locals
	{
		Locals::FP23DCanvas* Canvas;

		static bool IsWidgetEnabled()
		{
			//return GEditor->GetSelectedActors()->Num() != 0;
			return true;
		}

		static FReply OnButtonClick()
		{

			TESTINT = 5;


			return FReply::Handled();
		}

		static TSharedRef<SWidget> MakeButton(FText InLabel)
		{
			return SNew(SButton)
				.Text(InLabel)
				.OnClicked_Static(&Locals::OnButtonClick);
		}
	};

	const float Factor = 256.0f;

	SAssignNew(ToolkitWidget, SBorder)
		.HAlign(HAlign_Center)
		.Padding(1)
		.IsEnabled_Static(&Locals::IsWidgetEnabled)
		[
			SNew(SVerticalBox)
			+ SVerticalBox::Slot()
			.AutoHeight()
			.HAlign(HAlign_Center)
			//.Padding(50)
			[
				SNew(STextBlock)
				.AutoWrapText(true)
				.Text(LOCTEXT("HelperLabel", "Select some actors and move them around using buttons below"))
			]
			+ SVerticalBox::Slot()
				.HAlign(HAlign_Center)
				.AutoHeight()
				[
					Locals::MakeButton(LOCTEXT("NewLayerLabel", "Add New Layer"))
				]

		];
		
	FModeToolkit::Init(InitToolkitHost);
}

FName FP23DEdModeToolkit::GetToolkitFName() const
{
	return FName("P23DEdMode");
}

FText FP23DEdModeToolkit::GetBaseToolkitName() const
{
	return NSLOCTEXT("P23DEdModeToolkit", "DisplayName", "P23DEdMode Tool");
}

class FEdMode* FP23DEdModeToolkit::GetEditorMode() const
{
	return GLevelEditorModeTools().GetActiveMode(FP23DEdMode::EM_P23DEdModeId);
}

#undef LOCTEXT_NAMESPACE

It is giving me the following errors:

“TESTINT” undeclared identifier
TESTINT is not a type name, static or enumerator.

Where should I be putting new code? I would like a more useful struct but I get the same error regardless of the type I use. Any help would be appreciated.