Add this any way you know how, for oh-my-zsh clone it into your custom/plugins dir like so:
git clone https://github.com/fuzzylogiq/autopkg-zsh-completion $ZSH_CUSTOM/plugins/autopkg
Let oh-my-zsh know this is a plugin:
touch $ZSH_CUSTOM/plugins/autopkg/autopkg.plugin.zsh
And then add autopkg to your plugins in .zshrc
plugins=(autopkg ...)
Then you'll probably want to reload your shell:
exec $SHELL -l