Switched Ethernet has been, and will also be increasingly common in current and future real-time and embedded systems. The IEEE 802.1 working group has recently developed standards and technologies, commonly referred to as Time-Sensitive Networking (TSN), to enhance switched Ethernet with timeliness and dependability. We address, for the first time, the synthesis problem for the TSN frame preemption standards IEEE 802.3br-2016 and 802.1Qbu-2016 by introducing two new configuration parameters: flow to queue and queue to Express/Preemptable MAC interface assignments. We present an optimization framework to determine these configuration parameters with reliability as the optimization goal. Our proposed framework is shown to outperform commonly used priority-assignment as well as intuitive approaches.