Post-Install Script for Deployment Plan (OPS Center)

I am trying to create scripts to be run for the pre-install and post-install section of a Software Deployment / Update plan.  I am able to create the name of the script within the Deployment Plan creation process; however, I can not figure out where to actual create the contents of the script and have it be run when the deployment plan is assigned to an asset.  I can't find anything in the documentation that discusses this process or anything online with regard to this.

I can create an Operational Plan to include a script and add it to the Operation Step;however, I can not determine the means to create the script associated with the Execute Post-Install and Execute Pre-Install.