Auto installation ubuntu

I'm trying to create a complete auto installation iso which will install customized Ubuntu image.

I successfully created an auto installer image by using remastersys tool, however, it does not install grub. After installation completed (haven't rebooted, still working on live cd) I need to run Boot-Repair application to install grub properly.

My question is, how is it possible to run a script in live cd (or pen drive) after installation finished? So it can also automatically install grub.

Thanks in advance.