In addition to Richard Cresswells and dpbradleys answer: If you neither want to create a TNS name nor the '//123.45.67.89:1521/Test' input works (some configurations wont), you can put
(DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = 123.45.67.89)(PORT = 1521)) (CONNECT_DATA = (SID = TEST)(SERVER = DEDICATED)))
(as one line) into the 'database' section of the login dialog.