Hi clcote,
If you are trying to add certain songs to your Photos slideshow on your iPhone, if it is running iOS 15 or 16, follow "Make and play a slideshow" in Play videos and slideshows in the Photos app on iPhone. When you get to the music part, there should be a search icon on the top right to search for the music you want.
If you are using iOS 10, as there is no longer resources from Apple on Photos with iOS 10, you can see if there is a search icon when adding music, as there is in the most recent iOS, however, there are no resources to verify this.
If you are adding music to a slideshow in Photos on your Mac, see Create slideshows in Photos on Mac, "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."
Take care.