Callout Textarea event-onItemChange does not work

InterwovenIsFun

I have a callout textarea. A popup opens on the click of the button where i select data(which is in the form of checkboxes on the popup form). once i click on the "ok" button of the popup, the data that i selected in the popup is populated in the callout textarea on the datacapture form. once this textarea is populated, i have to populate another textbox in the same form. this has to be done without any button or anythng of that sort. onItemChange does not work in this case since i am not manually filling data in the textarea. inspite of that, the requirement is that i have to populate the textbox on the value change of the textarea. . .please can someone suggest me somethng. .

ps: onCallout wont work because on the click of the button, the textarea is blank. i have to do it once it is populated with data from the popup