I've created a module which extends the magento 2 checkout, where I've added another field according with the magento documentation:
This has worked and they are displayed on the frontend, however the validation causes a weird error. If I use:
<item name="validation" xsi:type="array">
<item name="required-entry" xsi:type="string">true</item>
As per the documentation, the field shows an asterisk (showing this is a required field), however upon submitting the form the error doesn't display and it doesn't prevent the form submitting.