Assuming you can purge manually , you can use my process.
After cleaning the keg, add a few liters water, close the lid and add some CO2 (just a few psi is enough), and drain the water through the "out" post and serving line. Repeat the process with Star San, making sure to roll the keg around to get all surfaces wet. You now have a clean, sanitized sealed keg with some CO2 pressure inside, and a clean, sanitized serving line ready for the next keg hook up. If you want or need to, you could add a step with Beer Line Cleaner or similar before the water step - probably a good idea every now and then.
When you are ready to fill the keg, simply open the lid, pour out the small amount of star san that didn't go up through the line, and siphon in your beer. Close the lid, pressurize with CO2, then purge the airspace. Repeat the purge 3 times and you've gotten rid of the vast majority of the O2.
Then force carbonate. I do that through the gas in post. As I am shaking the keg or rolling it on the floor back and forth, it doesn't matter really which post is used, but this way I don't have to change my connection hardware just to do the force carb.