How do i get my guitar to squeal on long notes?

Whenever I hit a note on my 1st, 2nd, or 3rd string that im going to hold, how do i make it squeal? i dont mean like pinch harmonics. That is why im wanting to knowi mean like for a few seconds it will sound normal, but then it starts to slowly squeal. a good example would be in the beginning of Hearts Burst into Fire, by Bullet For My Valentine. Also, if anyone here has covered this song or knows how, how could i get that tone in the begining when the distorted guitar comes in?