with (PATHTO.YOURTEXTFIELD_INSTANCE_NAME) {
    addEventListener(FocusEvent.FOCUS_IN, focusField);
    addEventListener(FocusEvent.FOCUS_OUT, blurField);
}

private function focusField(e:Event) {
    switch (e.currentTarget.name) {
        case "YOURTEXTFIELD_INSTANCE_NAME" :
        if(e.currentTarget.text == "YOUR DEFAULT MESSAGE") e.currentTarget.text = "";
        break;

    }
}
private function blurField(e:Event) {
    switch (e.currentTarget.name) {
        case "YOURTEXTFIELD_INSTANCE_NAME" :
        if(e.currentTarget.text == "") e.currentTarget.text = "YOUR DEFAULT MESSAGE";
        break;

    }
}

Paste this into your AS file to add focus and blur events to your text/form fields. If your field contains YOUR DEFAULT MESSAGE when focus is brought to it, it will erase itself, to be ready for text entry. If focus is lost while the field is completely empty, the default message will return.

AS3 Form Field focus/blur functions

form, post

ActionScript 3