Adding Apple Care for Device

See the following from AppleCare Products - Headphones - Apple

How to buy

1. Purchase AppleCare+ with your new headphones.
2. Or buy it within 60 days of your headphones purchase:

• • On your paired iPad or iPhone (go to Settings > Bluetooth > click on the ⓘ button for your headphones)
  • Online
  • At an Apple Store (inspection of headphones and proof of purchase are required)
  • By calling 800-275-2273 (requires you to provide proof of purchase)

The support article you came here from should provide you with the methods available to purchase an AppleCare+ plan for your Apple devices. Have you reviewed it yet? If so, and if it didn't help, I suggest that you contact Apple Support directly. To do so, simply select the "Get Support" link in the upper right-hand side of this forum's web page.