How to make a release with extra package

Hello everyone,

I have looked at documentations on how to buildworld and release scripts to make iso images. What I am not able to figure out is how to add a package to my release. In other words, I want to make an iso file for freebsd where when I install it the package htop for example is already there ready to use (i.e. no need to do pkg install htop) it is pre-installed with freebsd. Please help

Thank you