My English is certainly insufficient, but I did not really understand the procedure to follow with the two guides to UltimateHero And SilverScorpion. But I found a video for a glitch on YT that is clearer than the other YT video above. Look at the beginning, the process is very clear :
Go to the helicopter after the 1st mission. If Buck is not mentioned, return immediately to the last checkpoint. I got it on the 3rd try.