Accidentally, I put about 1/4 gallon of windshield fluid into the coolant tank. Then I drove it for about 5 kilometers, with the outside temperature of -3F/-19C. The windshield fluid light was still on, so when I opened the hood, I realized the mistake. What do you guys suggest? Definitely, I will clean up the overflow coolant tank, and put some antifreeze in it. But, do I need to flush the radiator or not?
I don't have experience with this, but in my opinion you should drain and refill it. Looks like somebody on Quora made the same mistake as you already. It would probably depend on the specific chemistry of your antifreeze and washer fluid, but it does seem like there are valid concerns of damaged seals and hoses, as well as degraded engine cooling. Certain radiators and blocks can also be finicky and corrode from certain chemicals. Ultimately, you shouldn't put anything in your cooling system other than proper engine coolants and distilled water.
That being said, since you mentioned it was very cold when you drove, and you only drove for 5 km (though, this will still depend on the speed you were driving at) I'd probably guess that your engine never got to temperature and therefore you didn't have any coolant circulating. So I don't think you'll have to worry about any damage for now. Depending on how comfortable you are with flushing your coolant, it may still be a good idea to do it.