What power adapter is required for the Apple Watch Magnetic Charging Dock

does not matter, as long as the connector fits, it's 5volts and minimum 5watt

Rudegar is correct. If you want to take advantage of the Series 7 Fast Charging ability, you'll want to buy a 20w USB C charging plug. Apple, Best Buy and Amazon all sell one. I particularly like the Anker Nano 20w USB C plug, which Best Buy sells for about $16.

I'll second the lobster's recommendation on the Anker Nano.