but the client instead puts a page blocked screen instead

That happens if you use the "focus address field" option in the new tab start page extension.

So, what you need to do is install the new tab extension that you want, goto the URL opera://extensions, turn on developer mode, click "details" for the extension, and take note of its ID.

Then, in the address field, you enter chrome-extension://theidyoufound/manifest.json. When the page loads, you look at the "newtab" value under the "chrome_url_overrides" object to see what HTML page the extension uses for its start page. It might say newtab.html for example.

Then, with that info, the full URL to the extension's start page is chrome-extension://theidyoufound/newtab.html. This full URL is what you put in the "New Tab Start Page" extension's settings.

See this post for an example.

Just note that this method works for most new tab extensions, but not all. Also, for the New Tab Start Page extension to be able to override the start page, you must not hide the speed dial and search box on Opera's start page. Leave Opera's start page settings alone.

@vastaway said in Opera GX New Tab Extensions:

Even with "Install Chrome Extensions"

That extension isn't needed anymore to install Chrome extensions. Opera has built-in support for the Chrome web store.