I am sorry about that. (I was also when I first came to SketchUp.) The makers of SketchUp seem to have purposely made SketchUp unCAD-like as much as they could.
This “specify the edge start point” is an often asked for feature, that has not yet been given to us.
What people are doing is drawing a unused temporary edge that end at the start point you want (by specifying in the Measurements box,) then continuing the tool drawing the actual edge by specifying it’s end point. When done, go back and erase the first unneeded edge.
There is a sample “linetool.rb” in the Examples extension, that might allow start point entry, or it could be tweaked if you know programming.
EDIT: Just checked the code of the sample “linetool.rb”, and it mimics the native C edgetool.
From the code, a comment just inside the tool’s onUserText() callback:
# We only accept input when the state is 1 (i.e. getting the second point)
… so the example would need tweaking.