Rated 5 out of 5 stars
Does not work in the latest version of Thunderbird 78.4.0, since the option to add text is missing when you go to 'Tools' - 'Add-on Options'.
Thanks for the feedback. It indeed works as described.
This was a change in Thunderbird (on which I has no influence ... I don't know why this was changed ...). Now you have to go to 'Tools' - 'Add-ons' - find the entry 'Standardtexter' - click on the 3 points on the right side - choose 'Options'.