How can I summon a charged creeper?

You can do this with the minecraft:become_charged spawn event:

/summon creeper ~ ~ ~ minecraft:become_charged

If you want to also give it a custom name, put the name after the spawn event:

/summon creeper ~ ~ ~ minecraft:become_charged "Charged Creeper"

Note: Only one spawn event can be used when summoning a mob, so you cannot use other spawn events such as minecraft:start_exploding when summoning charged creepers.