We study the extraction of the $B\to K^*$
transition form factors on the lattice by using the nonrelativistic
effective field theory in a finite volume. The determination of the matrix element at
the resonance pole is considered in the case of coupled-channels. It is shown that, in the limit of
the infinitely narrow width, the expected result for the matrix element is reproduced. The results,
contained in this contribution, are discussed at length in Ref. [1].