Sign in to the your Synology account,
then from the options in the left hand column, select "Security";
The title of the main section of the page will change to "Security", scroll down to the section "Security Key", then click on "Enable";

If you have not yet added any authentication factors you will be presented with the following messages, select 

In order to add a Fido key as an authentication factor you will now need to select "Security Key", then click 

The following confirmation screen will now show, click
;

In order to create a passkey using your Fido key, click 

Select the option "Windows Hello or external security key";

Then select the option "Security key";

If you key is PIN protected you will need to supply the PIN code then click
;

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;

Once you have touched the Fido2 Key will need to verify it is you that is about to add this new device as an authentication method.
Enter your password then click
;

Provide a suitable name for your security key, then click
;

You will now be notified that your Fido2 key is enabled, click
;

The Fido key will now be added as an active authentication device (example below), and is ready to be used when you next log in to your account.
