Add new field in registration form from custom plugin

[ctgraham]

You’ll want to avoid editing the core files (like lib/pkp/schemas/user.json or lib/pkp/classes/core/DataObject.inc.php) in favor of calling hooks within your plugin.

Have you seen this thread?

How can I add a required field in New User Registration Form?

I managed to find the right hook. It was publicprofileform::execute which worked for me. Thanks

It contains references to several examples.