Did you try passing a yaml dictionnary as the help message suggests?
`ros2 topic pub /test_topic std_msgs/String "data: Hello World"`

Copied from:
```$ ros2 topic pub --help
usage: ros2 topic pub [-h] topic_name message_type [values]

Publish a message to a topic

positional arguments:
  topic_name    Name of the ROS topic to publish to (e.g. '/chatter')
  message_type  Type of the ROS message (e.g. 'std_msgs/String')
  values        Values to fill the message with in YAML format (e.g. "data:
                Hello World"), otherwise the message will be published with
                default values
```





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