The ticket number, for example #123, that is input in custom field is transcribed into the ticket that is specified with a custom field.
If you have any issues, create a new ticket.
Download the zipped source from here.
Make the egg package:
python setup.py bdist_egg
Copy the egg into your Trac plugins directory:
cp dist/*.egg /where/to/trac/plugins/
To activate the plugin, edit your ../conf/trac.ini file:
[components] relevantticketplugin.relevantticket.* = enabled