If another user creates a topic and grants you permissions to publish messages, you can publish text or JSON messages to the topic.
You have obtained the URN of the topic granted to you. For details about obtaining the URN of a topic, see section 3.1 Creating a Topic.
on the upper left to select the desired region and project.The SMN console is displayed.
Clicking
beside Topic URN only enables you to select a topic you created, instead of a topic to which you are granted permission.

For more details, see sections 3.6.2 Publishing a Text Message and 3.6.3 Publishing a JSON Message.