show_message isn't showing me anything

[DavidTCS]

I'm just starting out trying to modify an existing plugin in a very small way ...
What do I need to do to have show_message() display a message?

The code has

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$this->rc = rcmail::get_instance();
and lots of code that is working, and I have added after the working code

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$this->rc->output->show_message('Message text','confirmation');
but I am not seeing the message.

If I can ever get this working, the message will say something more useful!

I'm probably missing something really obvious ...

[SKaero]

It would need to be done from a API call from the front end code in order for the message to be shown. Can you share more of the code so we can have a better idea how the code is running?