How to add extra field in Submit ticket page

For custom fields, please refer to the documentation: https://getawesomesupport.com/documentation/awesome-support/custom-fields/

Regarding agent assignment, it is not possible to have multiple agents for the same ticket.

One more question can we add extra fields on register page?

This is currently not possible.

Actually you can do it but it requires some custom coding.

You can hook anything you want to the registration form by using the wpas_after_registration_fields action hook, but then you need to handle the data processing.

For processing the data, you can hook onto the wpas_register_account_after action hook. This hook will pass two parameters: $user_id and $args.

The $args var contains the data inserted in the user profile. You can still access the $_POST data in your function in order to handle extra data passed in the form.