Active vibration control is crucial for mitigating harmful resonant vibrations in structures subjected to harmonic loads. While antiresonant (zero-placement) methods are effective for this purpose, existing state-feedback solutions require full state measurement, and output-feedback approaches often prioritize resonance assignment over direct harmonic cancellation. This work bridges the gap by proposing a novel systematic design for a Proportional-Derivative (PD) output-feedback controller to achieve antiresonance. The method first computes a homogeneous stabilizing gain solution. It then leverages the parametrization of all antiresonant solutions as a constraint within a genetic algorithm optimization. The algorithm optimizes both the stability margin, characterized by an Ms-disk criterion, and the number of encirclements of the critical point in the complex plane, as assessed by the Generalized Nyquist Stability Criterion. The proposed approach provides a practical, optimized output-feedback strategy for precise rejection of harmonic disturbances, as demonstrated through three numerical examples from real-world applications. The results confirm the method's effectiveness in synthesizing stabilizing controllers that enforce antiresonance while ensuring robust stability margins.