Here is the best way I have discovered to plot a movable segment in Desmos. Take note that in order to create it I had to have prior knowledge of Cartesian coordinates and parameters. See below for more.
Here I have a version in GeoGebra where I have utilized the limitation of tools available to the student to just "plot a point" and "plot a segment". It is very simple. Make your own right here and now!
In my humble opinion, I could teach a lot of Geometry, starting with the simplest of notions, in GeoGebra, such that beginning students can "latch on" right away. This "latching on" would not be so quick in Desmos.
However, both pieces of software would be used in my classes!