Predefined List form field type php class location / registration

Question

I'm trying to set up a Predefined List form field type as per the docs here https://docs.joomla.org/Predefined_List_form_field_type

I have the XML <field name="_card_expiry_year" type="Myext.CardExpiryYearList" label="Expiry year" description="" />

And in the same folder the class MyextFormFieldCardExpiryYearList

But how do I register the class and get the form field to read the options from it?