LastPass Icon > Open My Vault > Account Settings > Multifactor Options.
Then, click the Edit icon next to Yubico.
Connect your YubiKey to your computer's USB port.
Then, focus (click) the
YubiKey #1 field. Then, press the button on your YubiKey device. A string of dots will appear.
Finally, click Update and enter your LastPass master password.
You're all set up!
When LastPass prompts you for multi-factor authentication, you can now use your YubiKey like you would for MFA with any other service. Enjoy!