I'm not sure exactly your intent, but there are two addons: standardtexter and textinsert/textblocks, that appear to do what you wish. That is, insert prewritten text into an email message.

David -- Thank You! I decided on 'Text Insert/Text Blocks'. I like the 'short-cut' text-string [Ctrl+D] feature. It seems to require enabling/disabling the add-on or restarting Thunderbird to reset/activate changes to the shortcut/insert table. Once I figured that out, it works fine.

Also, the [Ctrl+D] shortcut does NOT WORK IF [CAPS-LOCK] IS ON. Just turn it off before pressing [Ctrl+D].