You will need to implement your own custom for for this - there is no way to add it to the standard Aware IM forms. For details about custom forms please refer to the Programmer's reference Guide.
You will need to implement your own TextArea class that inherits from the EXT TextArea class and implements this functionality by overriding onKeyUp or something like that. Then you will need to use this class in your custom form.
Alternatively you can make changes in the Aware IM code directly, which will make it available in the standard forms without having to implement custom forms, but then you will have to merge your changes whenever you update.
Incidentally, we are planning to include this feature in one of the next releases.
