How To Force Users To Choose Service Provider?

How do we force a user to choose a "service provider". We did go to line 1215 in the class_app_shortcodes.php to change the text that said "no preference" to now read "Choose Office Location". This is a temporary fix but as it stands if a user ignores the drop down for choosing a provider then the appointment request still processes. We use the service providers as locations. Also, is there a way to shade all form fields until the service provider is chosen then the "select a service" becomes available and so forth.