How to add MP3 files to iCloud Music Library?

You would need to import the files into iTunes or the Apple Music App, and then let them match or upload them to the iCloud Music Library.

For this, the Sync Library option needs to be turned on in Preferences / Settings for the App you are using.

click here ➜ Import media from a computer to iTunes on PC - Apple Support

or here ➜ Import music from a computer to Apple Music on Windows - Apple Support

then here ➜ Use iCloud Music Library on iTunes on PC - Apple Support

or here ➜ Use Sync Library with your Apple Music subscription - Apple Support