It's the leaves closest to the trunk, see these pictures:
I would cut at the red line here:
You'll notice that after cutting back to the last leaf on the branch on the tree in the picture below, that triggered growth from the base of that leaf that, once that branch lignifies, I'll then begin pinching back as well:
That answers my question I have a willow leaf ficus. I think. I bought a tree from a guy in Puerto Rico this year he sent me.a Little ficus as a side thank you gift. Really nice guy.
I think it's a good idea to let the branch grow to the required thickness, then cut back and worry about creating secondary/tertiary branching. If you are constantly cutting it back to simulate more buds the branch is going to never thicken. I've noticed that my willow leafs start backbudding along the trunk and branches pretty easily, more so so than my microcarpa and burtt davyi.