Not sure what special attachment one would need to preform closed transfer. Just connect up CO2 tank to blow off tube at top of fermentor, attach liquid line post to line coming from racking port on fermentor, and let beer displace gas in corny keg. Set CO2 regulator for 2 or 3 PSI, open gas and racking port. I place keg on milk crate(and scale) below fermentor for gravity assist, but it works if both at same level also.
To minimize oxidation, pre purge keg with CO2, either with fermentation gas, or later by multiple purges or liquid displacement.
It helps to used scale if batch is greater than 5 gallons, so one can keep some headspace in the keg(s).