You can use a loader from the port in your photo, but need to fix the arms and it is not very practical.
It's better to use a valve to switch the supply from the link arms to your loader.
If you remove the standard cover you see a pipe which comes from the pump. The pipe may come off with the cover, but it's safer to try to keep the bottom end connected.
Connect a diverter in the same way as the cover you removed. (it may be wise to replace the rings)
You can also get diverters with an integral spool valve. Extra spools can be bolted onto the side of the block.
There are many types of valve, but the square blocks are more origional. The last photo is how I had my 135 with simple "trip" loader. The yellow lever switched between Link arms and loader, The Red lever was single spool to lift the loader and the white tap on the top allowed me to switch between the loader or rear external connection. If you already have the spool valves on the loader then you only need a simple diverter. To use the loader the quadrant needs to be set to constant pumping.
The port high up under the seat is a return port although it is often considered too narrow for some applications. Many use either a port in the cover where PTO handle is or the filler cap.
Second hand valves can be hard to find - but they are also available new.
Hope this helps - Mike