Place Points of Mouse Position along Handles.DrawLine

I'm trying to create a mesh creation tool where you can right-click on the line to add another vertex. I am having trouble transforming from screen space to the world space and injecting the point in the correct space in my line segment list.

This is essentially what I want to happen.


I'm currently trying to get the position it with this code but it keeps sending me wrong values something like (0,0,0) when I would be expecting something like (0.5f, 0f, 0f)

Vector3 pos = HandleUtility.ClosestPointToPolyLine(lvlMesh.GetVertices());