class NewFLdlg final : public WithMyAweasomeLayout<TopWindow>
{
public:
    NewFLdlg() {
        // This callback logic should be implemented here... (Not outside the class - I am not surre why we have public inheritance here)
        button.WhenAction = [=] {
            // this->fsp should be available here.
        }
    }    

    String GetFsp() const { return fsp; }

private:
    String fsp;
};

// Then in the main code
NewFLdlg dlg;
if (dlg.Execute() == IDOK) // Check if user close dialog with success - you need "OK" button for such operations...
{
    String fsp = dlg.GetFsp(); // Here you should have valid fsp with nice object related design.
}