If you want the capping station to hold cleaning fluid, turn off the printer, then turn it on untill you hear the pumps start clicking, then turn it off using the switch in the back... The pumps are now active, sealed, and the caps will not leak out the cleaning fluid. Or use the head wash cycle and unplug the printer. Same thing.
Or use an automotive brake hose clamp on the lines going into the waste bucket if you can reach them.
Just sliding out the head and pouring in fluid- won't work, as when the pumps are inactive, theyre open and everything will leak out into the waste bucket.