Autofill OTP to the TextField when I receive message iPhone app

You cannot access messages on ios and hence you cannot autofill the OTP in ios apps by reading sms.

The answers in the link that you have posted do not read the sms messages. They just create a custom url scheme and the otp is filled in when the user clicks the custom url in the sms.

UPDATE

From iOS 12 Apple will allow the support to read One Time Code(OTP - One Time Password) which you will get in the iPhone device.

iOS supports Password AutoFill on UITextField, UITextView, and any custom view that adopts the UITextInput protocol. System keyboard set the textContentType on it to .oneTimeCode

1) Using Code

singleFactorCodeTextField.textContentType = .oneTimeCode

2) Using Storyboard/XIB

Select UITextField/UITextView in storyboard/XIB click Click on Attribute inspector. Go to text input trait, click to Content type and select one time code and done.

The operating system will detect verification codes from Messages automatically with this UITextContentType set.

Check out the official documentation for Enabling Password AutoFill on a Text Input View