To use an expression, you need to create an selector of text animator with any transformation (Animate: >) in the text layer and add the (Add: >) Expression Selector to it. The expression must be inserted into the Amount parameter of the selector.

delay = 0.03; // Delay
freq = 3; // Frequency
amp = 60; // Amplitude
decay = 10; // The smaller the value, the more fluctuations
Speed = 0.8; // The smaller the value, the slower the animation
myDelay = delay*textIndex;
t = (time - inPoint)*Speed - myDelay;
if (t >= 0){
freq;
amp;
decay;
s = amp*Math.cos(freq*t*2*Math.PI)/Math.exp(decay*t);
[s,s] }else{
value
}