it doesn't show my already reddemed gift card as a payment method

[compact ferry]

how to add this as my payment method

[tacit oriole]

@compact ferry Thanks for posting! Please remember, support here is volunteer-based and not official. Be patient after posting, and don’t spam or ask in other channels.
If your issue is about accounts, billing, contact Xbox Support directly.
If your issue is related to an Enforcement action, no one here can help.

To help us help you, make sure to answer:

• What did you try to do and what happened? (Be specific—mention your Xbox model, PC version, Insider ring, or anything relevant.)
• **What have you tried to do to fix it already? **(List everything you’ve done to fix it.)
• Have you searched the error code or for similar problems? (Tell us what you found.)

If you don’t answer these, you might not get a response.

[opaque needle]

if the balance isn't showing on checkout then that would normally mean the gift card and store region do not match. Or you redeemed it too a other account.

You would need too reach out too support for billing issues.