If I want to add the additionalProperty under the Thing type, what should I do?

Your best approach is to use the schemadotorg_additional_mappings.module and apply https://schema.org/Product to all node mappings.

The schemadotorg_additional_mappings.settings.yml config would like something like...

default_additional_mappings:
  node--Thing:
    - Product
    - WebPage
default_properties:
  Product:
    - additionalProperty
  WebPage:
    - dateCreated
    - dateModified
    - inLanguage
    - name
    - primaryImageOfPage
    - relatedLink
    - significantLink

@jrockowitz Thank you very much! According to you tip, I configured on page /admin/config/schemadotorg/settings/types as the screenshots below:

You can see, I'd like to use custom_field as the field type of additionalProperty. However, when I create a schema.org content type as below, it's still the text(plain) field and I cannot change the type:

@jrockowitz Sorry to bother you. Would you pls have a look at the issue at #3?