I don’t have that machine but in general hydraulic circuits can be thought of as a loop. The pressure goes to the valve and then returns to the reservoir. Your valve should have a pressure line in and the return line. The return line is the “power beyond”.
You will take the old return line (now your power beyond) and go into your 3rd function valve. Then from there you will go out of the 3rd function and that will be the return line.
The third function kits are the electric over hydraulic valve, a switch that controls the valve that typically is on the FEL joystick, some hoses and a bracket. If you have a local shop that can make hoses and you can fab your own bracket typically you can save about 1/2 the cost of a kit. Scorpion makes nice joystick switches and the valves can be had from several places.