Making a custom form-tag required

I followed the instructions at https://contactform7.com/2015/01/10/adding-a-custom-form-tag/ and at https://contactform7.com/2015/02/27/using-values-from-a-form-tag/ to add a custom form-tag for an input field with an associated datalist. This is working fine, apart from I cannot see how to make the field required. I have registered two tag types – “datalist” and “datalist*”, but when I specify “datalist*” as the tag type in the form, there is no error displayed when the field is empty.

Do I need to add some custom validation? Be grateful for any pointers on this one ?? thank you for reading…