With those red extensions you're supposed to open the valve on the tub then screw the extension on, use a small amount of PTFE tape, you then use the valve at the top to seal the tub properly
I use one of
these. Unlike other screw-on extenders you can open & close the valve in the tub. So I only use the extender when inflating the tyre then keep the extender in my toolkit & use a valve
decal/pad to cover the valve hole.