You can create a passkey from the passkey settings page.
Please see the articles below for more information.
The procedure to set up a passkey is as follows:
① Access the passkey settings page from the URL below.
https://accounts.pixiv.net/passkeys
② You will be asked to confirm your pixiv account, and once that is done, you will be taken to the passkey settings page. Click on the "Create passkey" button to go to the passkey creation page.
③ Click the "Create" button on the passkey creation page.
④ Authentication may be required depending on the device you are using. Follow the on-screen instructions to complete your device authentication.
(Example for iOS)
(Example for Android)
⑤ Passkey has been created. You can edit the passkey name if you wish.
The passkey will be available the next time you log in using the same device or any device that has the passkey synced.