No, you should never put sealant into the coolant reservoir. Doing so could cause serious damage to your engine and result in expensive repairs or even replacement of the entire system. Instead, here are a few steps to take if you have a leak:
• Check for visible signs of leakage such as wet patches and puddles on the ground below the car.
• Inspect hoses and gaskets for wear or tear that may be causing leaks.
• If necessary, use a sealer specifically designed for this purpose.