Retrieves an extended field by the specified extended field key.
You can only call this method when authenticated as a Wix app or Wix user identity.
Extended field key.
When accessing contact data,
extended field values are available at info.extendedFields.items[key].
For example, if the key is "custom.patronus",
the value can be accessed at info.extendedFields.items["custom.patronus"].
key is generated when the extended field is created.
It can't be modified, even if displayName is updated.
The specified field.
This method doesn’t return any custom errors, but may return standard errors. Learn more about standard Wix errors.