If you're playing as a hero in a campaign who shouldn't die, then you can't run away. Otherwise, just wait for your unit's turn (neutral creatures can't surrender for money).
To activate auto-battle, go to the menu/settings and check the box next to auto-battle. However, I don't know if it will ask if you need auto-battle or always conduct auto-battles, as I don't use them because they cause unnecessary losses in my army.
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