Just open up one of the wireless remotes. Perhaps that will void your warranty on that particular wireless remote, but big woop. Or go buy a generic one that works with liftmaster, they don't cost very much.
Once it is open, you should see that there is a small switch of some sort that closes when you push the button. Attach the two wires from an Elk relay to the two sides of that switch. Have your relay close for 2 seconds or so and that will be the equivalent of pushing the button for 2 seconds. Provided you locate the remote within the range of the garage door, it will work. This is how I have my Elk connected to my garage doors which are also Chamberlain Liftmaster's. I actually have three overhead doors and used a 3 button remote connecting three elk relays to the three button contacts.
Edit: I just looked at the specs on your door and it uses the 373lm remote, this is the same as mine. It is a typical rolling code unit that can be had for $20 on amazon, or generics are half of that. I have several extra becuase I programmed my "homelink" on the car to open the doors