Block that deals damage within a radius when redstone activated

You could use commands to reach entites in radious.

Can someone give an example of the code blocks for this?

An example sure, with code blocks no.

List<LivingEntity> mobs = entity.world.getEntitiesWithinAABB(LivingEntity.class, entity.getBoundingBox().grow(20), null);

That gives an iterable list of mobs within 20 blocks. Not radius though.

Do I copy this and post it into a command code block?

That is a java code that can be put in a class. But it won't do anything without an iteration.

Now that custom snippets are back;

List<LivingEntity> mobs = entity.world.getEntitiesWithinAABB(LivingEntity.class, entity.getBoundingBox().grow(20), null);

use a World management code block "Execute "command" at x y z"

in the command text field enter something like "/effect @e[r=16] minecraft:instant_damage 1 5", this gives all entities within radius 16 blocks 5x instant damage effect for 1 second 

This is how I did it in 1.15.2

/effect give @e[distance=..2] minecraft:instant_damage 1 1