The "message" query returns a specific Message. The query requires a mandatory attribute for the id of the Message to retrieve.
The org.opencode4workspace.builders.MessageGraphQLQuery class has a simple static helper method, buildMessageGraphQueryWithMessageId(String). Similar methods are also available from WWClient and WWGraphQLEndpoint.
For greater control, create an ObjectDataSenderBuilder for the Message.ONE_MESSAGE_QUERY_OBJECT_NAME object, adding the Message id attribute, passing in Message fields and children. Instantiate a new MessageGraphQLQuery passing in the ObjectDataSenderBuilder as the parameter. Then pass this to WWClient.getMessageWithQuery().