brew/Library/Homebrew/cask/doc/cask_language_reference/stanzas/pkg.md

pkg

This stanza must always be accompanied by uninstall

The first argument to the pkg stanza should be a relative path to the .pkg file to be installed. For example:

pkg 'Unity.pkg'

Subsequent arguments to pkg are key/value pairs which modify the install process. Currently supported keys are:

• allow_untrusted: — pass -allowUntrusted to /usr/sbin/installer

Example (from alinof-timer.rb):

pkg 'AlinofTimer.pkg', allow_untrusted: true