I know this thread is 4 years old but im sure there are others looking for the answer, I was.
What you need to do is set wordwrap to the textfield component of the textinput
textinputname.textField.wordWrap = true;
walla multiline textInput, hope this helps
OK just realised what i have done :/ this is AS3 apologies for my noobness
Thaks, I was just looking for that