Hey Fred!
Thanks so much for getting in touch. In order to set the fields as required in your plugin’s form, the setting would actually need to be modified within your MailChimp account itself. If you go to your Lists page in MailChimp, then go to that list, then Settings > List fields and *|MERGE|* tags, you can set your desired fields to required.
Once you have them set to required in MailChimp, you’ll need to go into the MailChimp plugin’s Settings in WordPress and click “Refresh” next to the list selection drop down menu.
That should do the trick!
-Gabe
