Hello Sadashiva,

You could create onchange Catalog Client script for catalog item. where variable is your sub category.

in script write the script.

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading || newValue == '') {
        return;
    }

    var messgae  =  "This is a messgae " + newValue;

    g_form.showFieldMsg('location', messgae, 'info');

}

Thanks,

Sagar Pagar