Two methods. First:
- Your hand drawn line must be on a separate layer
- Draw a mask path over the center of the hand drawn line
- Set the stroke to use the mask path and make it wide enough to cover the entire line
- Animate the start or end point of the stroke
- Set the stroke to Reveal Original
(you could also use a stroked mask path on a separate layer as a track matte for your dashed line)
Second:
- Create your dashed line with a shape layer
- Add Trim Paths and animate either the start or end of the stroke