FFT analysis of brainstem response

I have a .mat file which is brainstem response waveform to auditory stimulus "da".I wanted to know the spectral amplitudes of F0,H2,H3,.H4 from the response.The following are the details

Sampling frequency- 20,000Hz

Start time: - 10.05 milliseconds,End time: 194.70 milliseconds

F0:100Hz, F1: 563-692Hz, F2:1453-1281Hz, F3:2510-2475Hz, F4:3285-3287Hz.

In an article the author has done FFT and have described the procedure in which the code was made attaching the screenshot of that too.

I wanted to analyze the spectral amplitudes of F0,H2,H3,H4.

Iam very naive to matlab and iam stuck with analysis for my thesis. Please help

Thank you