- This event is triggered when a user opts in to text messaging.
 - Pass 
nullforlanguageCode, unless the desired language is French then pass a String with value"fr". - A Wunderkind teammate will supply your 
accessCode. 
Wunderkind.shared.trackTextOptIn(phoneNumber: String, accessKey: String, languageCode: String?)Wunderkind.getInstance().trackTextOptIn(phoneNumber: String, accessKey: String, languageCode: String?)Wunderkind().trackTextOptIn(String phoneNumber, String accessKey, String? languageCode)Wunderkind.trackTextOptIn(phoneNumber: string, accessKey: string, languageCode?: string)