Claim your website by going to Google Merchant Center and click on Business information > Website > Claim Your Website
From the options near the bottom, click on HTML Tag and copy the meta tag
Log in to BigCommerce and go to Advanced Settings > Web Analytics, here enable Google Analytics under the General Settings
Click on Google Analytics tab and paste the meta tag in the tracking code section and save.
Go back to Google Merchant Center > Business information > Website > Claim Your Website and click on verify URL.