The item ID for Mythril Brick is 416
If there is no item ID, please use the item name to spawn.
This is the spawn command to give yourself Mythril Brick in Terraria. Terraria does not have a vanilla item spawn command and varies between singleplayer and multiplayer. For singleplayer you can use the inventory editor. For mutiplayer you can use a plugin or mod like TShock.
Example spawn commands that will work with TShock:
Copy the command below and paste it into your Terraria game or server admin console to obtain.