How to add a pop-up selection window?

Hi,

I know use below script, there will be an error window alert when @id is not null by using the <error> tag?

    <leave>

      <check expr="[@id] != ''">

        <error>You must specify a list.</error>

      </check>

    </leave>

But how a build a confirm window where we can choose "yes" or "no"?

Thanks,

Felix