That looks like the reed. There should be a screw/plug right next to where that came out. Look at the parts diagram above. What you pulled out is #1. You need to be looking for 3,4,5,6. In that diagram. You have to follow the “lines” at the end of each part and they lead to where it goes on the flow switch body. You can see that what you pulled out is right next to where the screw/plug is. They call it a screw on the parts list but it will look more like a bolt head or a plug with a slot to where a screwdriver fits.
My pressure washer isn’t at home right now or else I would snap a picture of mine and point to it. My fuel pump looks exactly like yours so I’m sure a lot of parts are similar.
If it doesn’t have the screw/plug next to the wire that might be the thermostat probe and possible you’re not looking at the flow switch. You need to look for the screw/plug next to the wire or reed.
