Hi Sue,
See link below which is for a very similar model to yours, the water connection method is the same.
The summerise, you will need a hozelock style connector on the end of your garden hose, (the type that you pull back the outer collar to disconnect) this pushes onto the water inlet on the back of the machine. The hose that came with the machine connects to the outlet on the front of the machine and the lance is fitted to the other end of this hose.
https://www.manualslib.com/manual/926299/Challenge-Xtreme-Ylq16s-150a.html?page=2#manual
Argos would have sold this machine, so calling their customer service number may help you obtain the original owners manual.
Rupert, April 2020