Can I enable mobile Purchase Alerts for the Additional Cardmember on my Account?

No. Purchase Alerts are intended to notify real-time Card activity for the specific Card holder. You can track and view your Additional Cardmember's (Supplementary Card) activity on the Amex App or on your Online Account. Any purchases made by the Additional Cardmember will have a badge with their initials next to the transaction details. The Additional Cardmember can set up their own Purchase Alerts on their device to be notified when their Card has been used.