Using Google Maps Tile Service with OpenLayers 5

Question

How to pass Google Maps API key to access tile service using OpenLayers 5.

Some google searches suggested me to set the url of tile layer as

http://mt.google.com/vt/lyrs=m&x={x}&y={y}&z={z}

Is this a legal way to access Google tile server data directly without an API key?

As per this forum, its a violation of Google Maps TOS.

If this is a violation, what should I need to do to meet their TOS.

I need to overlay some WMS data hosted on GeoServer over Google Map. I can do so using Google Maps tile server, but without using a Google API.