how do I download a song from apple music to a powerpoint slide

You almost certainly cannot do this.

Apple Music grants you the right to personally listen to and enjoy the music - as long as your subscription remains active.

You are not allowed to distribute or share this music.

Janet,

You will have to buy the song as a download.

If you are unsure of the difference between purchased downloads and Apple Music tracks, see here:

Add and download music from Apple Music or buy music from the iTunes Store - Apple Support

Whether you purchase the music or basically “rent” it using Apple Music, you still have no rights to distribute it.

The music is for your personal use.