How to show the PrintDialog just after showing the preview in FastReport?

I'm using Delphi to develop a desktop database application, and I've an invoice report made with FastReport, I know that I can use InvoiceReport.ShowReport to show it's preview. So I need to know how I can show the print dialog automatically just after showing the preview. The user then can print it or cancel the dialog

To show print dialog in Fast Report you simply call Print with PrintOptions.ShowDialog:=True. By default the preview form is shown modal, so the easiest solution is to change it and call Print:

InvoiceReport.PreviewOptions.Modal:=False;
InvoiceReport.PrintOptions.ShowDialog:=True;
InvoiceReport.ShowReport;
InvoiceReport.Print;

If you need to keep the preview form modal, the other option is to handle OnPreview event and call Print, but you have to postpone the procedure, so for example:

const 
  UM_PRINT_DIALOG = WM_USER+1;

...

procedure ShowPrintDialog(var Message: TMessage); message UM_PRINT_DIALOG;

...

procedure TForm1.Button1Click(Sender: TObject);
begin
  InvoiceReport.ShowReport;
  //if you want to show the report once it is fully rendered use:
  //InvoiceReport.PrepareReport;
  //InvoiceReport.ShowPreparedReport;
end;

procedure TForm1.InvoiceReportPreview(Sender: TObject);
begin
  PostMessage(Handle,UM_PRINT_DIALOG,0,0);
end;

procedure TForm1.ShowPrintDialog(var Message: TMessage);
begin
  InvoiceReport.PrintOptions.ShowDialog:=True;
  InvoiceReport.Print;
end;