When a phone visits a web page the web page can send SMS through your phone with an sms hyperlink. You can use something like this to send a pre-composed message through the original iPhone SMS app only. Notice the "&" before the body - it is not an error. Since iOS 12 I think, this is needed but if you use the hyperlink in android you can remove it and replace it with "?"...
B4X:
Dim sPhone As String = "+30210..........."
Main.App.OpenURL($"sms:${sPhone}&body=This is my message..."$)