I have heard some of the newer Slingshots have a sealed U-joint.. If you don't see a fitting like the one pictured below then yours is sealed..
You will need the needle fitting that I have pictured below to push in on the center of the grease fitting..
You can either pop the cup holder out and get to the fitting that way. Or you can lay under the SS after safely jacking it up and get to it that way..
I used a Lithium grease on mine.. You will actually take the needle and push it into this tiny hole in the grease fitting. Push slightly hard to keep the seal.
I pump it into the fitting until I can see it start to show up oozing out of the Universal joint caps.
Then I wipe any excess grease off. Sometime you will have to pump a little to much out of one cap to get the other to start coming out.