I can't actually make a "Send Later" button appear by installing this add-on.
EDIT: The developer is right; I apologize; I should have asked for help before just writing a bad review. With this add-on installed, I can indeed customize the toolbar in the Write window and permanently add a "Send Later" button, which then does exactly what is advertised and invokes the "Send Later" menu.
I'm changing my rating from 1 to 4 stars. If the developer can simply make it clear in the add-on description that the user still needs to customize the toolbar, I promise to change my rating again to 5 stars.
Thunderbird 68 doesn't support the prior possibility to automatically add the button into the toolbar. So you've to use the toolbars customization dialog to add the button into the toolbar.
Thanks for this nice 1 star rating.