Interface Submersible water pump with microcontroller using Contactor Relay
If you want to keep the existing manual Start/Stop buttons (highly recommended!), you will need your MCU to control two relays.  The Start relay Normally Open contacts should be wired in parallel with the existing Start button, and the Stop relay Normally Closed contact wired in series with the existing Stop button.  The MCU should pulse either relay for 100 mS or so (whatever time is required for reliable operation...)