This cant be accomplished in the wizard, but you can add it manually.
You can apply Required not blank validation to the on change event of the select list, and use the show hide layer response.
I am sending a sample to demonstrate this.
to add validation to the on change event, go to Window -> Behaviors
Click on the select list in desing view, then click the plus button on the behavior window and select WA Validation Toolkit -> Client validation -> Required / Not Blank / Selection Made.
do the same for adding the show hide layer response. click the plus button on the behavior window and select WA Validation Toolkit -> Show hide Layer.