representable functors and finitely generated algebras

Question

I'm bothered by a confusing statment on representable functors. Let $k\rm Alg$ be the category of finitely generated $k$-algebra ($k$ is a field) and $F:k\rm Alg\to Set$ a functor. Let $B={\rm Nat}(F,U)$ where $U$ is the forgetful functor $U:k\rm Alg\to Set$, i.e. the functor represented by $k[t]$. Then $B$ is a $k$-algebra and $F$ is isomorphic to ${\rm Hom}(B,-)$. I have trouble to figure out the element of $F(B)$ corresponds to ${\rm id}:B\to B$. Can any one tell me what is it?