Just pop in the command set to always active then on top of the command block add a stone block a redstone block and an activator rail.
you must throw down a wither skeleton skull and a nether star for him to be summoned
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https://www.youtube.com/watch?v=2WvE8-gWyBo&list=PLGF7mhMsEcMn0oalwlzl71Zta8YHMlgGP&index=4