Forcing Drop Down Selection

I have set up a custom form in my checkout for users to select an option from a list of options provided via a drop down menu.  Basically a list of approved locations they can choose from.

I thought it worked perfectly but have now realized many skipped the dropdown choices by selecting nothing and then hitting continue; and this allowed them to submit the order without choosing a location.

Squarespace does not force this selection apparently so now I have many people I need to individually reach out to and get their choice from.

Is there any way I can force them to make a selection for future orders?