VS Codium server can't install extensions

The VS Codium server we plan to use for our MVC(&8621 (closed)) doesn't seem to support installing VS Code extensions (from open-vsx) out of the box.

extension overview searching for extensions
Screenshot_2022-08-23_at_15.14.15 Screenshot_2022-08-23_at_15.14.23

Without extensions we can't install linters, which are a hard requirement for an MVC (see @ericschurter's comment)

Edited by Tomas Vik