It's the icing on the cake.
Here, is on the cake an adjunct or a complement of the noun icing?
A complement is something that is required to complete a grammatical structure.
An adjunct is something that adds something, but is not essential for the grammar.
In a sentence "It is a red cat", a red cat is a complement, because "It is" is not complete without it. But red is an adjunct, because "It is a cat" is complete.
So you can answer your own question if you can say whether "It's the icing" is grammatically complete or not.