I need to dispute a transaction

If you want to file a dispute, you would do that with Cash App. If you want to request a refund, you would do that with the merchant, Apple in this case.

You first step is to determine if the charge is for a subscription. If it is a subscription, it’s a valid charge and does not need your authorization to withdraw funds. Please review the information in the Apple Support article I linked below.

If you want to cancel a subscription from Apple - Apple Support

If it’s not a subscription, it’s possible a purchase was made on your account. You can review the information in the article below and find all recent charges.

If you don't recognize a charge - Apple Support

Once the subscription is canceled, you may request a refund from Apple. The article linked below explains how. If the request is approved, funds are usually returned in 5 to 7 business days, but may take up to 30 days.

Request a refund for apps or content that you bought from Apple - Apple Support

If it’s not a subscription, it might be a fraudulent charge, and you can dispute the charge with Cash App. Call the phone number on the back of the Cash App debit card.

At anytime you’re welcome to reach out to Apple Support at 1-800-MY-APPLE

If you mean "Cash App", I don't see that as a valid payment method for iTunes or the App Store anywhere. So, you're going to need to dispute this through whatever method Cash App offers.

Charged by whom? To what payment instrument?