Show route line for map view

How can I show route lines on map views?

I have start and stop lat/long columns. Each shows a pindrop of each point. How can I show the route between the points? I’ve been searching polylines and in the Route Optimization Sample App there is a way and I’ve spent days trying to break down quite how but I don’t know how to write Appscript.. I have gotten as fas as setting up an API and linking to a script but I can’t seem to breakdown the script and adapt it to my needs ..

Google Maps example of what I would like:

Route Optimization Sample App Screenshot

https://developers.google.com/maps/documentation/javascript/examples/polyline-simple