First, get a temperature sensor of some kind to put into same room as A/C and connect to home assistant.
I personally use a ZWave multisensor mounted in the room.
Then, turn your A/C knobs to the coldest, always on setting.
Last, use the generic thermostat component and link the temperature sensor and the smart plug connected to the A/C together so it can control the A/C temperature by turning the A/C power on (since it is set to coldest setting your A/C thermostat will always tell A/C to run).