So while setting up the goals in Google Analytics, if you Chargebee's Thank You Page is already configured, then under the 'Destination' section, the following text needs to be entered - '/thank_you'.

This would help the goal to get triggered when the destination reaches the configured URL.

Also, incase of a custom based Redirect URL configuration {own 'Thank You' page}, its appropriate path as a part of the Goal URL should be entered.

Here's a document which gives an overview on how to set up these goals.