How do I get a refund for accidentally adding money to my Apple account instead of Apple Cash?

Check your Apple Account balance - Apple Support

If cash refunds are required by law

You can’t redeem or return Apple Gift Cards, App Store Gift Cards, or unused Apple Account balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support to request a refund. When you contact Apple, include the following information:

• The primary email address that you use to sign in to your Apple Account
• The refund amount
• Your current shipping address

click here ➜ contact Apple Support

If you added money from a debit card, then contact Apple directly and they may be able to reverse the transaction, so it goes back to the bank that issued the card.

Contact Apple Support - Apple Support