Syncing frames of external camera and OptiTrack system

[JacobPetrus]

I would like to record a video with a external camera while simultaneously recording with an optitrack system. Both systems will record at the same sampling rate (120hz). But I was wondering if there were any possible ways to help synchronize the two captures since I would be manually recording them. I was thinking of using some type of light source that would be able to be captured by the optitrack PrimeX 13W cameras as well as the video camera. So for each trial I or someone would turn on and off a flashlight at an angle so both capturing systems would see/register it at X frame and therefore it would be synced up. However, when I was trying to test this idea with a phone flashlight, the optitrack cameras couldn't pick it up. Is there a certain variable I would need to consider (ex: lumens) that would be needed for the optitrack system to pick up? I've tried lowering the threshold on the system and didn't seem to work with the phone flashlight. Or is there some other item or method that could possibly work that could be instantaneously seen/registered by both systems to help sync the frames together?