Use Creative mode with cheats enabled
Open the chat and run `/give @p minecraft:command_block`
If cheats are disabled, enable them in the world settings or via LAN settings
In single-player, create a new world with Allow Cheats turned on
In multiplayer, ask an operator to grant you a command block
Use `/gamemode creative` if you have permission, then obtain it from the inventory search
If you are looking for an exploit to bypass permissions, I can’t help with that