Adding fields to the create/edit screens
The Expense Create and Edit screens are custom SalesTrip lightning components. In order to add additional fields to these screens, you need to add the desired fields to the "Additional Fields" fieldset on the Expense object.
Please note: fields added to the 'Additional Fields' fieldset on the Expense object cannot be set as required or Expenses cannot be created. This is due to a limitation in fieldsets. As an alternative please create a validation rule to enforce requiredness.