If you mean
Hold L to Show Secret ID
5216F4CC 98087128
0216F4D0 F9FAF699
94000130 FDFF0000
0216F4D0 0C006900
D2000000 00000000
then you are supposed to hold L as you click your name on the menu.
So you are supposed to go like this:
start the game with the code
once you are ingame, press X
click on your name (trainer card and badges) while holding L
click on trainer card (relrease L)
check your ID, it should now be your secret ID