It depends on how long you want the light to be on and how many flashes you want them to flash.
If just one short flash then:
MyLight.sendCommand(HSBType::RED)
Thread::sleep(1000) // nothing longer
MyLight.sendCommand(OFF)
If you want the light to stay on for longer than one second you need to get more complicated:
MyLight.sendCommand(HSBType::RED)
createTimer(now.plusSeconds(5), [| MyLight.sendCommand(OFF)])
If you want to flash repeatedly for a long time see Loop rule (in OH2) which lets you flash Morse Code.