//THIS JUST SAVED YOU HOURS OF RESEARCH -> UPVOTE :)
I just added a string.xml file to my android project with client id of a web client

strings.xml content(app/source/main/res/values/string.xml):
  YOUR_WEB_CLIENT_ID 

So in google cloud console go to credentials(https://console.cloud.google.com/apis/credentials) and get your web client's client id.