WebTextArea.InsertText
From Xojo Documentation
Method
WebTextArea.InsertText(OneBasedPosition as Integer, Text as String)
Supported for all project types and targets.
Supported for all project types and targets.
Inserts the passed Text to the current Text starting at position OneBasedPosition.
Example
Inserts text at the beginning of the text area:
TextArea1.InsertText(1, "First line" + EndOfLine)