If you have an AJAX button in your form, a nice way of adding javascript is to use an IAjaxCallDecorator
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form.add(new AjaxButton("removeButton") {
    @Override
    protected IAjaxCallDecorator getAjaxCallDecorator() {
        return new AjaxPreprocessingCallDecorator(super.getAjaxCallDecorator()) {
        private static final long serialVersionUID = 1L;
            @Override
            public CharSequence preDecorateScript(CharSequence script) {
                return "if(!confirm('Are you sure you want to delete this?')) return false;" + script;
            }
        };
    }
}
