Sign in to the your PayPal account by supplying your username and password,, and then you will be prompted to insert your security key;

Insert you Fido2 key into an available USB port,  Select "Security key", and you will prompted to touch the key  (in the case of a Fido key with a fingerprint reader you will need to swipe your finger on the key);

Once you have touched the key you will then be logged in to your PayPal account.