There are a few different places that you can find your Spotify Artist ID.
In the Spotify app:
Click the "..." next to the "Follow" button
Click Copy Spotify URI
Paste the Spotify URI into the Search Box. Delete "spotify:artist:" leaving only the Spotify ID.
From a browser:
Copy everything after "https://open.spotify.com/artist/". This is the Spotify ID. It should look similar to: "4o4tLTIJ3eWMFerz73atcT"
Paste the Artist ID into the Stem Artist Information Search field!