The Android Auto app now supports the "OK Google" hotword: surprisingly, it didn't before
In order to issue commands to the app, you will need to either wait for v2.0.6427 of the app to hit your device or you can follow the source link we've provided down below and sideload it manually yourself. Either way, you will be able to interact with the app by starting off with the "OK Google" chant.
One would think that an app aimed at making driving safer would offer this crucial functionality from Day One, but Google has a history of rolling out barely finished apps. Just check Google Allo — the app that had the unique chance of bundling all of Big G's messaging services into one and becoming a worthy rival of iMessage for Android is becoming less and less popular, just recently tanking down to number 200 in terms of overall user downloads in the Play Store's charts.
Better late than never, eh?