It is not possible to insert a link into an E-Mail because the insert link button is greyed out. My geary version is 40.0 (tags/gnome-40.0-0-ge561775c1-dirty).
This seems to be already fixed in gitlab gnome org/GNOME/geary/-/issues/1118 (Sorry can’t post links).
I can’t reproduce it. I’m able to insert or edit links just fine.
Yes, that issue was fixed and merged over a year ago. It’s included in the repo package. You should have
geary 1:40.0-8 installed.
User error. I have to select text to make the link button usable.