If you're referring to a separate flash unit triggered by an optical slave, then I'm afraid there might be no way for it to be triggered by the "flash" of the L920. The optical slave was designed to recognize short bursts* of intense light, i.e. from a xenon flash which is what you find in cameras with built-in flash. The "flash" of the L920 is not a "real" flash but is rather a simpler LED designed to assist or aid in brightening up an under-lit subject.
Hope this helps.
*- Flash bursts are so short that they are sometimes used in ballistic photography - freezing bullets in mid-air... but these are specialized flash units with bursts at around 1/1,000,000th of a second. Most flash units are at 1/800 - 1/2000 of a second. LED flashes found on phones do not approach those speeds.