If you're talking about the Laser Designator in Rannoch... if you're targeting the Reaper base... you need to stand on the ledge, and if you get a targeting circle, you can lase the target (hold the fire button until the columns converge for the strike).

If you're facing the actual Reaper... you need to wait until the targeting circle is available, then you hold the fire button (although you can hold the aim button to slightly lock on faster, I extremely discourage it until the slow-motion part, because you need to roll side-to-side so you don't get burned.