Scripting a maintenance plan

When you create job using maintenance plan, a job is automatically created in SQL Server Agent under Jobs. So now go to SQL Server Agent, expand jobs, and look for your job. Right click on it and select Script job as... Drop and recreate to... New query editor window.

Edit:

You can export the maintenance plans as well. You need to connect to Integration Services of the instance. Expand the stored packages then MSDB then expand Maintenance plans. Right click on MP and select a export package. Then you can move the package to another SQL Server.