Hi, if this is a vented system, you can seal the header tank lid with clingfilm or similar, put a jug or tray under the joint to repair, then open it up slowly, you'll get a bit out but it will create a suction and hold the water back.
once you've repaired the joint, just pour the antifreeze back in to the header tank (obviously you'd want the system cold and turned off first, it,s a good idea to shut the valve at the other end of the rad first.
if it's the gland on the radiator spindle, try just tightening the gland nut a bit first, you can also repack these with twisted ptfe to reseal.
hth
paul