The IR Laser ought to be used the same way that you employ any other optic in the game. Modern IR lasers such as the ATPIAL are typically employed looking over the top of the weapon with momentary switch activation when the user decides to aim and fire (or designate) at a target.
Likewise when a weapon mounted light is employed, this coincides with aiming towards the target area and momentarily activating the light to fire.
I suggest that in addition to the toggle activation of the laser/light, an option to momentarily activate the laser/light be available as a keybinding (e.g ALT+L).
I also suggest that the activation of the laser in this case NOT use other optics on the weapon as it wouldn't be desirable to use both Laser and optic concurrently. However shifting the view to look over the bore of the weapon would be helpful for the player to judge the trajectory of their bullets.