Here is how you can add a simple action to open a folder in VSCodium within Nemo file manager.

1. Create a new file in ~/.local/share/nemo/actions/ and name the file vscodium.nemo_action
2. Open the file in a text editor
3. Copy the code below into the file

[Nemo Action]
Name=Open in VSCodium
Comment=Open VSCodium in the selected folder
Exec=codium %F
Icon-Name=vscodium
Selection=Any
Extensions=dir;

4. Save the changes made to the file
5. Now when you right click inside a folder in Nemo, it will show an option "Open in VSCodium" and when you click this option, it will launch VSCodium using the currently directory as the workspace.

This can be modified to work with Visual Studio Code (Which is closed source unlike VSCodium) by editing codium %F to code %F