Question: The problem with applyrule and substitution z*conjugate(z)=x

I am trying to make a substitution:

> restart;
> f1 := conjugate(z)^2*z^2;
> f2 := applyrule(z*conjugate(z) = x, f1);

however the result is

f2 := conjugate(z)^2*z^2

Do you have any ideas how to solve the problem? Thank you.

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