This paper presents a novel flat-top directional modulation (DM) synthesis approach utilizing constant amplitude antenna arrays. Unlike traditional modulation schemes, the proposed method leverages phase-only synthesis to generate DM patterns, offering several distinct advantages. Notably, it enables control over the peak-to-average power ratio (PAPR), facilitates the design of flat-top beam patterns, and allows for adjustable beamwidth to optimize bit error rate (BER) performance. These capabilities enhance the adaptability and efficiency of DM systems, making them particularly well-suited for integrated sensing and communication (ISAC) scenarios requiring robust beam scanning. Through simulation results, we validate the effectiveness of the proposed approach in achieving controlled BER beamwidth and reliable DM performance. This work lays a foundation for its application in advanced wireless communication and radar systems, where precise beam control and power efficiency are critical.