You can preset your Spotify playlist (no matter yours or Spotify created) with the following steps:
1. Play your playlist in Spotify App
2. Stream it to WiiM Mini via Spotify connect
3. Open the WiiM Home App > Tap on the Mini play bar and go to "Now Playing UI" > Tap the three dots button (...) below the track progress bar > Tap the preset button and select which preset number you prefer.