The ICommandManager interface is managing commands. So you can use it to get and execute commands.
You can get this from an IServer object.

Importing the package

Link to importing-the-package

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.command.ICommandManager;

<table spaces-before="0"> <tr> <th> ZenGetter </th>
</tr> <tr> <td> commands </td>
  Map<String, <a href="/Vanilla/Commands/ICommand/">ICommand</a>\>
</tr> </table>

To add your own command, look at CommandEvent