In addition to kyze's answer you could use the notes- url syntax. If the notes- client is properly installed, then it will react to urls that are in the format:
notes://Servername/dbname.nsf?Open
or (if the database is in a subdirectory of the data directory
notes://Servername/subdirectory/dbname.nsf?Open
If you want to access a local database, then use
notes:///names.nsf?Open
(this example opens you private addressbook)