Make a select form field required

When adding a form rule for select (or multiple select) fields, these need to be applied on the client side (not server side). Here’s an example of how to add a rule for a select field:

        $mform->addRule(
            'selectfield', 
            get_string("selectfieldrequiredmessage", "local_yourplugin"), 
            'required',
            '',
            'client'
        );   

The key is the 5th parameter above which is set to client instead of the default of server.

For more help, see the Moodle Forms library developer documentation.