Why not just drain and disconnect at both ends, lift the rad off the wall, take outside and just stick the hose in the end and run until it flows clear??
Otherwise you'd need a 3/4" to 15mm coupler with a bit of 15mm pipe or another 3/4" union lockshield valve with a bit of 15mm pipe in the bottom, or take that tail out and put a 15mm tail in. or ......
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