Extract the onset of pulses in praat

Question

I am currently wirting a script to calculte VOT of consonants in English

Tier 3 : phoneme

Tier 4 : CV The goal is to detect in the interval of Tier 4 the onset of the pulses. Here is a part of the code :

thisSound$ = selected$("Sound")
thisTextGrid$ = selected$("TextGrid")


select TextGrid 'thisTextGrid$'
numberOfCV = Get number of intervals: 4
appendInfoLine: "There are ", numberOfCV, " intervals."

for i from 1 to numberOfCV
  appendInfoLine: i
 select TextGrid 'thisTextGrid$'
    thisCV$ = Get label of interval: 4, i
appendInfoLine: thisCV$

When I try the command "pulse listing" it doesn't work...

Can someone help me with that ?

Many thanks !