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I don't see this working, I have the config set up but there is no button or option.
Edit: by turning on the Composition Toolbar in the compose window I can see the button, but it's grayed out and has no effect. I still can't find the shortcut, what is STRG?
Tbird Windows 78.4.3

Questa recensione riguarda una versione precedente (1.3) del componente aggiuntivo. 

Note that the default shortcut was incorrectly advertised here as ALT-W, it's STRG-SHIFT-W again. Might clash with other addons, though.

The button can be added when opening the customization dialog in the compose window (right-clock on button bar).

Otherwise drop me an email, ideally already attaching some further information (TB version, error console log).