Unfortunately you can’t directly access individual vertices (points) on lines/curves/shapes…even though seemingly EVERYBODY (including me) asks Adobe for it!
But, all is not lost.
You can use the Effect > Generate > Beam effect instead.
1. Put your footage in a comp.
2. Add a null
3. Add a black solid
4. Track the footage, with the Null set as your motion target
5. On the black solid layer, add the Beam effect
6. Make sure you can see the Position keyframes on the Null layer – there should be LOTS of keyframes (from the motion tracker)
7. Open up the black solid’s Beam effect in the timeline
8. Alt-click the Ending Point to add an expression
9. Pick-whip the Null’s position
10. Set the Length of the beam to 100%
11. Set the thickness to something thin, the colour to white and the start point to wherever looks good.
12. Do a RAM preview and enjoy
13. Do your best impression of a lie-detecting cockney geezah!