You may have a tank or a bag it dependent on model etc. If its a bag is a straight swap over and reset.
If it has a tank you need to read remaining fluid level with diagbox, then top up with the correct fluid, hose connector colour is the ID for fluid (i would expect a light plug tab which requires Powerflex fluid).
The bag or tank are located in front of the left rear wheel under a cover, Bag just plugs in tank you will see a approx 10mm hose and push fit connector that you disconnect and connect the filling kit to.
You need to work out the correct amount,diagbox/planet shows the tank capacity and you just subtract was is left in the tank to get the top up value. Do not fill to overflowing (as if like my DS it will take the best part of a litre more than needed) as there is no level sensor so it will says its runout again when it thinks the normal volume has been used even if you have put extra in.
The easiest way to reset counters is to use the maintenance option in diagbox and select additive refill and follow the instructions. You can do it via engine or additive ECU options but dependant on ECU the options can move around