Log in to your ZeroTier Account, then click on the submenu option "Account";

You then click on the link next to the Authentication prompt: "Manage Account" (see below);

Expand the section "Account security", then click on the link "Signing in";

Next, scroll down the central section of the web page until you reach the section "Passkey", then click on "Set up Passkey";

You are now prompted to register your Fido2 key, insert the key into an available USB port then click ;

When asked where to save this passkey, select the option "Security Key", then click ;

You are now notified that access to google will be prepared with your Fido2 security key - click  to proceed to the next step;

You will now be prompted to enter the PIN code for your Fido2 key.

At the prompt "Security Key PIN", enter the PIN code, then click . 

(in the case of a Fido key with a fingerprint reader you will need to swipe your finger on the key);

You are now prompted to touch the key (to prove you are present and giving permission for the key to be used).

Touch the necessary area on your Fido key;

You are then notified that the passkey has been saved, click ;

You will now be prompted to name this new authentication method.

At the prompt "Security key name", provide a name for your device then click ;

Your authentication options will be updated to include your newly registered Fido key;

Your Fido2 key will now be ready to use when you next log in to your ZeroTier account.