You can follow these directions to use points to cover your card’s charges on the Amex Platinum card:
Visit American Express’s Membership Rewards website.
Log into your American Express account.
After logging in, you’ll be redirected to the Membership Rewards dashboard. Hover over the button near the top of the page that says Earn and Redeem and click Cover your Card Charges.
You’ll see a list of recent charges on your Amex Platinum card.
Select all of the charges you want to use points for and click Redeem Now at the bottom of the page.
It might take up to 48 hours for the request to show up online.