the customer portal currently doesn’t show that a field is mandatory until they try and submit their incident/request. Can this be changed as it is not a good customer experience?
I assume you are talking about Service Desk Customer Portal. Optional fields are marked as (optional). All other fields are required. You can add help line to each field in Customer Portal where you can emphasize that field is required with exlamation icon (!) + text or other formatting of your choice by using wiki markup.