The ticket number(ex.#123) that is input in custom field is transcribed into the ticket that is specified with 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 egg into your trac plugins dir.
cp dist/*.egg /where/to/trac/plugins/
To activate the plugin, edit conf/trac.ini:
[components] relevantticketplugin.relevantticket.* = enabled