To create a Facebook SSO login, you'll need to follow these steps:
Login into Meta for developers and click on your FB App - > Login with FB.
Under Valid OAuth Redirect URIs add your Webstore's domain using this format :
https://YOUR_STORE_SUB_DOMAIN.YOUR_STORE_DOMAIN.com/login
Then under Allowed Domains for the JavaScript SDK add your Webstore's domain using this format :
https://YOUR_STORE_SUB_DOMAIN.YOUR_STORE_DOMAIN.com/