$\{a^n : n \geq 1\}$ is a subset of $\{b^ma^k : m \geq 0, k \geq 0\}$.
So, their union will be : $\{b^ma^k : m \geq 0, k \geq 0\}$
Since, the question is asking for an NFA, we do not need to draw all transitions. So, here it is:
The missing links lead to a dead state.