JJ's solution is a common way of getting a grapple powered. However, since your tractor has the special joystick with rabbit/turtle buttons instead of a knob on top of a shaft, it's a bit harder to mechanize because mounting the momentary contact switch is a pain. Also, you really need a solenoid to help isolate the double selector valve that JJ mentioned, instead of just running power directly from a switch on the joystick. It's also not a bad idea to have diodes for arc suppression from the selector valves inductive load.
If all this is 'Greek' to you, it might be good to talk to your dealer about a 3rd function kit. WR Long sells a 3rd function kit that mounts into your system hydraulics rather than divert the loader curl circuit. I have a remote valve for my grapple. I run 1/4" lines from the rear remote up to my grapple and operate it with a lever on the right fender near the joystick. The nice thing about this setup is that I can also hook in a 3PH log splitter on the rear and use the remote for that when needed. If you want to do the job yourself, adding a rear remote for around $750 and then hoses and fittings for the grapple around $200 more. You'll always have to have the hoses/fittings anyhow. The total cost is about $1000 with remotes, but might be a a couple hundred dollars less with the diverter valves if you can do that job yourself.
I would sure recommend talking to your dealer's service department about what they suggest.