Audit tables in different databases
You can do that with the help of the additional module dblink. Install once per database (the one where the trigger is executed):

CREATE EXTENSION dblink;


You need superuser privileges (in the database cluster where the trigger runs) for some of the involved functionality.

Since you want to write to the external database, you will probably want to use dblink_exec(). The manual has code examples.

Or try a search here for more dblink examples.

Note that you cannot ROLLBACK what you have written with dblink to an external database. A local exception after the external write does not undo what's written in the foreign database.