Hi,
In Sierra you had to add a Google Messages Account in Messages then go to System Preferences > Internet accounts and enable the Mail Option to allow the Internet account to do both.
In High Sierra it is now different.
The Internet account for Google does not have a Messages option.
You have to Add a New Account (Other Account) and then choose Messages Account.
Here you can add your Google Name.
Set Automatically find Server and Port as Google are now only using the gmail.com server and trying to set talk.google.com does not work.
A long time ago I set the "Less Secure Apps" option in the Google Account then Security Page.
High Sierra tells me at every login but it works.
9:33 pm Sunday; March 4, 2018
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)