invoices.lay:
LAYOUT(CustomersWindowLayout, 1112, 476)
	ITEM(SqlArray, CustArray, SetFrame(ThinInsetFrame()).LeftPosZ(436, 656).TopPosZ(28, 420))
        .
        .
        .
	ITEM(Button, btnAddCustomer, SetLabel(t_("Add")).SetFont(SansSerifZ(16)).SetFrame(ButtonFrame()).LeftPosZ(24, 88).TopPosZ(352, 28))
END_LAYOUT

customers.cpp:
CustomersWindow::CustomersWindow() {
	String DBFile;
	String configfile = ConfigFile();
	CtrlLayout(*this, "Customers");
	
	btnAddCustomer <<= THISBACK(btnAddCustomerClick); // <-- only line added in this function since last successful run
	
... 

void CustomersWindow::btnAddCustomerClick()
{
	PromptOK("Click Add");
	return;
}

customers.h:
struct CustomersWindow : WithCustomersWindowLayout<TopWindow>
{
	String GetOutputDirectory();
	String SelectDB();
	
	String	DBFile;
	String OutputDirectory;
	String configfile = ConfigFile();
	String cfg;

	FileSel selectdbwin;
	FileSel selectodirwin;
	
public:
	typedef CustomersWindow CLASSNAME;
	CustomersWindow();
	void btnAddCustomerClick();
	void Paint ( Draw& w )
	{
		w.DrawRect ( GetSize(), Color ( 204, 255, 255 ) ); // <= enter your background color here
	}

	
};