How can I take a String input and publish it to a topic as a String?

edit

ROS String.msg isn't the same as user input string - you have to use it like here, the part with the callback:

void timer_callback()
{
  auto message = std_msgs::msg::String();
  message.data = "Hello, world! " + std::to_string(count_++);
  RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.data.c_str());
  publisher_->publish(message);
}

to use string from the user (so std_msgs::msg::String(), message data field is your C++ string).

The online documentation is here: https://docs.ros.org/en/humble/

To see some example code on how to publish a message, click on the "Tutorials" link, then the "Writing a simple publisher and subscriber (C++)" link.