This document will outline how to require a specific field be filled out when a new patron is being created.
Navigate to the patron add form through Patrons > New patron dropdown and choose any patron category from the dropdown. Specific patron categories can vary from site to site, so the options on the dropdown menu on your own site might look different from the one shown in this example.

As it is possible to add or remove items from the Patron Registration form, the content may vary from site to site, however, it should look something like this:
All fields with a Required in red beside the box must be filled in before the record can be saved. Mandatory Fields
Go to Koha administration > System Preferences and search for BorrowerMandatoryField.
This will bring up several system preferences with the same phrase in it. The system preference to enable this functionality is called BorrowerMandatoryField.
This system preference includes a box with a lock icon. Clicking in this box will create a popup which will show a list of all the borrower fields which can be marked as mandatory.
To use this feature, click into the small box to the left hand side of the name of the field to be made mandatory in the borrower registration form. If a field has a white tick on a blue background to the left hand side, that indicates that the field is required on the add patron form. If there is an empty white box to the left hand side, that indicates that the field is optional on the add patron form.
For example, if the primary email field should be mandatory when creating a new patron, click the box to the left hand side of that field name in this system preference,
Before marking the email field as mandatory:
Once the desired field has been marked as mandatory scroll down the page and click Save.