To add a point at the current mouse position to a line in a frame called MyFrame:
var Pos = GetMousePosition();
MyFrame.AddPoint( Pos );
To draw a line and then insert a point into the middle of that line:
// Draw a line
MyFrame.DrawLine( 100, 100, 100, 300 );
// Insert a point after the first
MyFrame.AddPoint( 0, 200, 200 );