postinst is in network-manager-openvpn 1.1.93-1ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14  | #!/bin/sh
set -e
if [ "$1" = configure ]; then
    # create system user required to run the openvpn daemon unprivileged
    adduser --quiet --system \
            --home /var/lib/openvpn/chroot \
            --gecos "NetworkManager OpenVPN" \
            --group nm-openvpn
    # create directory used for running the openvpn daemon in a chroot
    install -o nm-openvpn -g nm-openvpn -d /var/lib/openvpn/chroot/tmp
fi
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