Currently VSCODEEXT is only available on Visual Studio Code's official marketplace site, which means it can only be directly used by people who are using Microsoft's official Visual Studio Code distribution.

The MIT-licensed Visual Studio Code redistributions like Code-OSS, VSCodium, code-server are not allowed to use VSCode's official marketplace, thus Open VSX Registry (https://open-vsx.org/) is used. Sadly, currently users cannot download Qt's VSCODEEXT from Open VSX Registry.

It's possible but not that easy to download the VSCODEEXT extensions as vsix files and load them manually for Code-OSS/code-server users, since the official marketplace doesn't offer a simple way to download the extension as vsix file.

Thus, I suggest uploading VSCODEEXT extensions to Open VSX Registry.