You can use the redirection method as follow for this case (by detecting the user agent being chrome ios):
https://www.facebook.com/dialog/oauth?client_id={app-id}&redirect_uri={redirect-uri}
See more info here https://developers.facebook.com/docs/facebook-login/login-flow-for-web-no-jssdk/
Remark: I personnaly use the server OAuth in that case but this should do the trick and is quite simple