To use the marketplace extension for Microsoft, we need to edit the product.json file located in the installation directory:

• on MacOS: /System/Volumes/Data/Applications/VSCodium.app/Contents/Resources/app/product.json
• on Linux: /usr/share/codium/resources/app/product.json

  "extensionsGallery": {
  "serviceUrl": "https://marketplace.visualstudio.com/_apis/public/gallery",
  "cacheUrl": "https://vscode.blob.core.windows.net/gallery/index",
  "itemUrl": "https://marketplace.visualstudio.com/items",
  "controlUrl": "",
  "recommendationsUrl": ""
  },

  Check the link for more.

With the installation of codium with snap, the files are read-only; so it is not possible to change the product.json. The workaround is to install the .deb package.