That example simply creates a list of random numbers and then prepends those with "CH ".
If I understand your request, you could create a definition that asks for 2 points and that puts an annotation at a third point. Native Grasshopper 1 doesn’t deal much with annotations, though, and you’d have to use a third party plug-in.
Are you aware that you can modify the Rhino annotation style to include a prefix? If you only need the text, you could explode the dimension and delete the lines and arrows…
-wim