How do I find my first digit code of my serial number iTunes card

See -> How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

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Redeem a gift card with your iPhone, iPad, or iPod touch

1. Find the 16-digit code on the back of the card. For some gift cards, you might need to peel or gently scratch off the label to see the code.
2. On your iPhone, iPad, or iPod touch, open the App Store app.
3. At the top of the screen, tap the sign-in button  or your photo.
4. Tap Redeem Gift Card or Code. If you don’t see Redeem Gift Card or Code, sign in with your Apple ID.
5. Tap Use Camera and follow the instructions on the screen. If you have trouble redeeming the card, tap Enter Code Manually, then follow the instructions on the screen.
6. Tap Done.