When given access to the Helix Rift missions, you must outscore the minimum score you are given after you gather enough data to locate the lost Assassin. Say you have to have a score of 500 to find the Assassin's location. You will then use your remaining time before the portal closes to gain above, perhaps, 1000 in data score.
Note: Freeing the Assassin gains you a ton of data score, so when grinding for this, make saving the Assassin after you find their location the top priority for you.