how do i upload music for photos slideshow?

If you add it to the Music library as Yer_Man suggested you then can access it in the slideshow this way:

Essentially like we did in an iPhoto library.

It will depend on your system version, which songs from your Music Library can be used in a slideshow.

Currently, on macOS 13.2, I can only use purchased songs or audio files without digital rights assigned, not Apple Music songs, and the songs have to be downloaded from the cloud.

Add it to the Music App Library is a simple way.