Let $\mathbb{K}_0 \subset \mathbb{K}$ be two tori (subtori of $(S^1)^n$), and suppose that $\mathbb{K}_0$ is of codimension $1$ in $\mathbb{K}$. We denote $EG \to BG$ the universal principal bundle associated with a Lie group $G$, and will consider cohomology with coefficients in $\mathbb{C}$.

I am trying to understand the classifying map of the circle bundle
$$
B\mathbb{K}_0 \overset{\mathbb{K} / \mathbb{K}_0}{\longrightarrow} B\mathbb{K},
$$
or more precisely its induced map in cohomology.

Let $eu$ denote the Euler class of this bundle. Then $H^*(B(\mathbb{K} / \mathbb{K}_0), \mathbb{C}) = \mathbb{C}[eu]$. Thus, the classifying map in cohomology is of the form
$$
f : \mathbb{C}[eu] \to H^*(B \mathbb{K}, \mathbb{C}).
$$

My question is the following: what is the element $f(eu)$ ?

Thanks to all for your help!