Go into the "PuTTY Reconfiguration" dialog, select "Terminal Keyboard" and set "The Function keys and keypad" to VT400. The DO key is then the "-" key on the top right hand corner of the numeric keypad. Be careful not to hit "Pause/Break" just above it - it's sent as ^Z.
The DO key is a standard LK-series function key found on LK201 keyboard and the VT220 and later terminals, and specifically is the F16 key, and F16 is encoded as:
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