How to get post code for my iPhone 📱

Do you mean Postcode? - the code at the bottom of your real address which helps the post office sort your letters?

I think you probably mean Passcode, the four or six number code you enter to unlock access to your iPhone.

If you have forgotten this you will have to erase your phone, and if possible restore from a backup made before the code was set. (The code only exists on the phone: neither Apple nor anyone else can know what it is.) Please see:

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

Where are you seeing a request for a post code in order to open the phone ? Do you mean the phone's passcode, or postcode on your iTunes account (which is tied to your billing address) ?