Dennis @ PerfectForms
We prefer to do all of our check mandatories behind the next page button or if you are at the end of your form, the submit button. Use a regular button called “next” or “submit” rather than a submit or page button so behaviors can be placed on when button is clicked.
Use a check mandatory behavior and add your drop-down to it. Make sure that none of the drop-down choices is selected (in the properties for the drop-down)or the check mandatory object will see that it has been filled out.