Mark Frequency as required and set default

Our users are getting caught up with the Frequency button. Because it does not have an asterisk, our users skip that field when submitting a request. The problem is that if they are submitting a request for multiple days, the system will not accept their request unless they select a feature in the Frequency field. I am wondering if you make that a required field, will that solve their problem? I tried to find the setting to change it myself, but do not see it.