Window air conditioners are not meant to be fixed and are most often tossed and replaced.
However They are fixable, often only needing a low cost part or a refrigerant recharge to get them working again.
Most window A/C units do not come with a service port Schrader valve and need one to be installed.
This is a easy process which only needs a piercing valve kit.
You can also solder on a valve but it is not necessary and comes down to personal preference.
Wire brush or emery cloth to clean the area.
A Bullet Piercing Valve.
Bullet Piercing Valves can be bought at most air conditioning part supply stores or online.
Bullet Piercing Valve on Amazon
Supco BPV21 Bullet Piercing Valve
The first step is to find a spot on the copper refrigerant line that will be easily accessible and clean it with a wire brush or emery cloth.
Get the valve and hold one side on.
Attach the other side.
Tighten down the outer screws evenly.
After the outer screws are in place the center can be screwed in which will go into the line piercing it.
You can now attach gauges on the the unit and test it or refill as you would any other A/C unit.
There are some HVAC technicians who prefer to solder on a valve but in my experience a screw on valve works just as good.
This is of course debatable and is simply a preference that changes from tech to tech.
If you are very good at soldering and can do it quickly and reliably than soldering a valve on may be best.
If you just need a quick port to add refrigerant or do some test than a screw on piercing valve will work fine.
Screw on type valves can become loose over time and 4-5 years may need to be tightened.
Window A/C units that are fixed are usually already 3-5 years old, if not older.
A screw on valve will more than likley outlast the units working life span.