Log in to your X account by supplying your account name, but instead of being asked for your password, you will asked to sign in with your passkey.
Select the option "Security Key", then click 
;
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 the button on the Fido key you will be logged in to your X account.