postinst is in systemd-journal-remote 237-3ubuntu10.
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 | #!/bin/sh
set -e
adduser --quiet --system \
--home /run/systemd --no-create-home \
--gecos "systemd Journal Remote" \
--group systemd-journal-remote
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