Using USB-TTL converter

Serial.print goto PA9.

git clone https://github.com/nopnop2002/Arduino-STM32-Ethernet
cd Arduino-STM32-Ethernet/UdpBroadcastServer
pio run -e bluepill_f103c8 -t upload

Using STM Virtual COM port

Serial.print goto USB Virtual COM port.

git clone https://github.com/nopnop2002/Arduino-STM32-Ethernet
cd Arduino-STM32-Ethernet/UdpBroadcastServer
pio run -e bluepill_f103c8_usbcon -t upload && pio device monitor -b 115200 -p /dev/ttyACM0

Screen Shot

UDP Broadcast Client

python3 udp-transfer.py