I read this somewhere and It got me wondering, was it something that was inherited from ancient times or was it something that was granted to them by the emperor?
As a point of order, the Prince-Electors of Brandenburg were elevated to Kings in Prussia in 1701 by Emperor Leopold I and later to King of Prussia in 1772, after Friedrich II took over Royal Prussia in the First Partition of Poland. The HRE was only dissolved in 1806, so the Hohenzollern monarchy would have to be included.
Now I'll grant that Prussia lay outside the borders of the empire (although Brandenburg certainly did not), but since it was the emperor himself who elevated the Hohenzollerns to the level of king, I would think it would have to be included.