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Your turtlebot's URDF should already have joints defined which implicitly define the base_scan -> base_link transformation. These should all be fixed joints so it will be a static transform. As long as you have a robot_state_publisher node using the URDF, that transformation will be published on /tf_static.

If you need to perform the frame conversion somewhere in your code to get the instantaneous positions in the base_link frame, you can look at tf2_ros's buffer_interface. But many applications (e.g. rviz) will be able to do these transformations automatically if the robot_state_publisher is running.