logic – Does any finite L-Structure has its k-catecory theory? (where k is of course the structure’s cardinality)

How to show that for any finite language L and finite L-structure A, there is an L-sentence b, such that for all L-structure B, $Bmodels b iff Bcong A$?

I searched for many first-order logic notes but didn’t find anything to make this seems solvable.