How to insert Gazebo plugin in an existing World Model?

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OK, from the tutorials I learned how to make a Gazebo Plugin. I successfully made the plugin and got the '.so' file.

I have an existing ROS package, which has world files and a lib folder with many '.so' files. How do I add my plugin to this existing package? I tried simple copying 'my_model.so' file in lib folder and modifying the respective <plugin ... filename="my_model.so".. > command.

But it did not seems to work?