evaluation – How to assign the expression of 2 variables evaluating it at the same time

My question concerns the assignment of the resulting expression after the evaluation of a function.

So what I want:
FOCF[x, y] : = D[F[f[F[f[x, y], X],
where he immediately evaluates the right side, making it a conditional expression.

How do I do it now:
I begin by entering: D[F[f[F[f[x, y], X].
then, I manually copy the resulting output into
FOCF[x, y] : = "copied terms".

Is there a way to do this? I've already tried using Evaluate[]but it did not seem to work. Please, let me know.