If you suspect that a fraudulent transaction has occurred on your card, it is recommended that you first block the card to prevent any further transactions. This can be done through the same channels as reporting a lost/stolen card.
Once your card has been blocked, you can call the Call Center on the back of your card. A customer service associate will guide you through the process of submitting the fraudulent transaction claim.
Please keep in mind that fraud disputes can take some time as they must be fully investigated and as such any refund owed will not be immediate. You will be required to provide supporting evidence to substantiate your fraudulent transaction claim.