How to get postcode in apple id?

I can't help wondering whether you, and some of the other posters here, mean passcode - the four or six digit number you used to lock your individual device (nothing to do with your Apple ID) and which you need to unlock it.

If you have forgotten your passcode, Apple cannot help you since it is is stored only on the device: you will have to erase it. Please see:

https://support.apple.com/HT204306

Of course, if you really do mean postcode, it's part of your physical address in some countries and you will have to find out what it is from your postal service.