1 - Log in to the Duo Admin Panel and click 2FA Devices in the left sidebar, then click Hardware Tokens.

2 - Click on the serial number of a token to access the token's properties page, e.g 10001002 

3 - On the token's properties page, scroll down to the Users table and click the Attach User button.

4 - Select a Duo user from the drop-down list and click Attach.

5 - The token's properties page now lists the attached user.