funny you should mention this about Karcher. Was using mine yesterday (it has been kept in garage) and the pressure has more or less gone!
In answer to the original question - I used to attach the hose to the kitchen sink tap and put it out of the window at my old house when i had no outside tap. The attachment basically had a big wind up jubillee clip on that held tight onto the tap, then fitted the hose, then fitted the karcher to the hose outside.