i forget how, I did on my last save so I know it's possible
Auto-Mode can be triggered at any time there isn't an enemy too close by pressing the L-button on the Switch (it won't trigger if an enemy is nearby). Auto-Mode then takes control, showing a white circle around the rescue team leader.
So, just press L when an enemy isn't nearby.
Source: https://www.usgamer.net/articles/pokemon-mystery-dungeon-rescue-team-dx-auto-mode-explained
