Suppose $A,B\in\mathbb{R}$ and $M=A+iB$ for $M\in\mathbb{C}.$ What condition must $A$ and $B$ be to satisfy $M$ is unitary.

Question

So far, I have AA^H=AA^T-iAB^T+iBA^T+BB^T. I have a feeling that A and B must be orthogonal. I just can't grasp what the other two terms must be. Thank you in advance.