Packages can be installed automatically at creation time!
Specify the pkglist property at creation time, which should point to a text file containing one package name per line. Please note you need to have Internet connection for this to work as pkg install will try to get the packages from online repositories.
Example:
Create a pkgs.txt file and add package names to it.
pkgs.txt:
nginx tmux
Now simply create a jail and supply the pkgs.txt file:
iocage create pkglist=/path-to/pkgs.txt tag=myjail
This will install nginx and tmux in the newly created jail.