King of Prussia, the answer to the question, was never a title held by Hans-Adam I. His official titles were Prince of Liechtenstein, Duke of Troppau, and Jägerndorf, Count of Rietberg, Lord of Schellenberg, and Vaduz. Hans-Adam I reigned as the reigning prince of Liechtenstein from 1989 until his abdication in 2004.
The Noble Reign of Hans-Adam I: Test Your Knowledge on the Prince of Liechtenstein