Here's the link to use to get Fastpass+ lunch reservations at the Magic Kingdom's Be Our Guest restaurant:
A couple other thing to note:
1 BoG FP+ lunch does not go against your 3 original FPP attraction reservations, nor does it count against any day-of Fastpasses you make.
2 FP+ for BoG lunch does not open up at the 30/60 day mark like normal FPP. Historically it has been opening up about half way through the month prior to your travel month (but of course, the dates are sporadic and this could change at any time).
3 BoG lunch FP+ reservations do not show up in My Disney Experience. You'll have to remember it (for now).
You'll need your resort reservation number to make this reservation. Here's how to find that number:
1) Log in to My Disney Experience
2) Click on "Plan and Manage"
3) Click on "My Reservations and Tickets"
4) Place your mouse cursor over the "Make a payment" link. Don't click anything - just put the cursor over the link.
A small text box will appear that has the 12 digits embedded.
Also, if you start the online check in process, you can find the number in the URL.