From the VirtualBox main screen, select "MX Linux" and click on Machine -> Tools. Scroll down to the "Snapshots" section.
Click on "Take" to take a snapshot of the current state. Use the name "Installed MX Linux" for this snapshot.
Now you have saved the initial post-installation state of your MX Linux virtual machine. If you ever want to or need to return your virtual machine to this state, you can restore this snapshot rather than reinstall MX Linux.