How do I add song from music library to a photos slide show

After you create a slideshow project with "File > Create > Slideshow > Photos", click the "music" button in the sidebar. Now you can select the Music Library instead of the Theme songs and add songs to the sound track by clicking them. Remove the Theme song manually with the delete key that Photos has already added to "Selected Songs". You can only select songs that are not dimmed. Protected songs with digital rights attached cannot be used. Also songs that need downloading from iCloud. If you are using iCloud with Music, force a download from iCloud in Music before you try to use the songs. See: Create slideshows in Photos on Mac – Apple Support (UK)

Note: If you don’t see music from your Music library listed, close Photos, then open Music and sign in if you haven’t already. With Music open, open Photos again, then select your slideshow. Click the Music button and choose Music from the pop-up menu to see your music list appear.