Try the Commands Troubleshooting and Help page if you get stuff with server errors.
Minecraft Enderman NBT Data Tags
Using the summon command to spawn in a minecraft enderman is simple, take the command and enter into chat and presto you have an enderman. Endermen can carry a blocks and can be summoned carrying blocks.
How to summon an Enderman carrying a block
To set the block an enderman carries use the carriedBlockState, with the Name property. Endermen can only carry blocks, and can not carry items. You can summon an enderman with block in hands with the below command.
/summon minecraft:enderman ~ ~ ~ {carriedBlockState:{Name:dirt}}
/summon minecraft:enderman ~ ~ ~ {carriedBlockState:{Name:redstone_block}}
/summon minecraft:enderman ~ ~ ~ {carriedBlockState:{Name:diamond_block}}
How to stop an Enderman disappearing
To prevent a enderman teleporting set the NoAI tag, this prevents enderman from moving.
/summon minecraft:enderman ~ ~ ~ {carriedBlockState:{Name:diamond_block}, NoAI:1}
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If you're looking for a particular summon mob generator you can link to these specific versions of the command generator.