How to add a custom tag (or search field) to a contact in apple contacts

Not so much a question as a workaround to share. If you want to add a custom search tag (field) for your contacts, go to Contacts/Preferences/Template (on a Mac).

then select add field if "Related Name" is selectable - do so, if it's grayed out then it's already accessible from the contact form.

Select "mother" or any other available title and at the bottom of the list select "Custom", add the desired new reference "tag"/field. Add as many as you'd like. Now each time you create a new contact or edit an existing one you'll have those tags as an options to fill in. Since the field is searchable it makes sub categorising contacts much easier.