Log in to your Shopify account, but instead of entering your email address, click "Sign in with passkey";
When asked where to save this passkey, select the option "Security Key", then click 
;
At this point you will be asked to provide the PIN code that protects your Fido2 key;
Enter the PIN code for your key then click 
, and you will be asked to touch the button on your key (if your model has a fingerprint reader you may be asked to provide a finger swipe instead);
After touching your security key you will be logged in to your Shopify account