Opera doesn't allow extensions to override the start page.

However, as a workaround, also install https://chromewebstore.google.com/detail/new-tab-start-page/mjfahldkpjhcnfbbmdhpjolcjpcfhcpj and in its settings, set the custom URL to Aurora's new tab page, which is:

chrome-extension://lcjggfamlfhhaggffljddjjlcgidknhg/newTab/newTab.html

Then it'll work except the address field of the new tab won't be blank and won't be focused. That's as good as it gets.