Flash MX 2004.
text.variableName
Property; a string that contains the name of the variable associated with the Text object. This property works only with dynamic or input text; it generates a warning if used with other text types.
This property is supported only in ActionScript 1.0 and ActionScript 2.0.
The following example sets the variable name of the selected text box to firstName:
fl.getDocumentDOM().selection[0].variableName = "firstName";