Depending on how cold it's going to get.. But in general the safety precausion taken for frost is put anything at least with 80 cm depth. As is for ponds not to freeze solid and give the fish a safe place to hibernate as is for frost prevention for subterranian water piping. But that is for very extreme cold winters. The coldest winter i ever experienced in my lifetime was about 50 cm ice on the lake behind my house. And that was an average of - 18°c for weeks.
So if you are planning to dig it in than make a box, bottom and side panels out of 4cm thick stryrofoam and top it off with 4cm thick styrofoam lid.. But than again, if such a small water volume is covered and dark for such a long periode nothing will survive and it will surface again after the winter as a stinking muck, all rotting organics need to be cleaned out anyway. Some plants can survive a mild winter, but definitively need light for that. I even had some Potamogeton gayi survive in an outdoor tub last winter.
And if you plan it to keep above ground, what i guess is your idea and what i would do. Why would you dig in such a nice ornament as a ceramic sink. 🙂 If you want to prevent it from water logging from bellow than don't put it with the base on the ground and elevate it a few cm with placing a brick at each corner so it has ventilation all around.. But than you still need to drain it. Would be a shame for such a nice sink to take the risk and find it cracked one morning, or just empty and leaking after the frost..