Put it just after the threat is revealed, then follow it with the FNAF jumpscare scream for the payoff, or lead into it with Enemy Spotted so the warning comes first.
About this sound
This is a very short voice clip of a Half-Life scientist letting out a nervous, high-pitched 'eeeh' in alarm. At just over a fifth of a second, it lands as a sudden squeak of fear rather than a full scream. It feels comic and startled, the kind of panicked reaction that makes a tense moment feel silly instead of serious.