Little help with ADS: how to measure S param at certain bandwidth

Hello there, I am exploring ADS and I'm trying to setup my measurements sheet for my simulation.
For a certain frequency, let it be fo, I want to measure S21 (or any other S-parameter) at fo+10% and fo-10%. How can I do that using equations?

i1=find_index(freq, (1-10/100)*fo)
i2=find_index(freq, fo)
i3=find_index(freq, (1+10/100)*fo)

S21[i1], S21[i2], S21[i3]

Thanks for the tip, it worked nicely for me.