In order to override the startup command of a Buildpack you can use an extension less file with one line of text named Procfile. The Procfile was popular in Heroku the originators of The-Twelve-Factor-App. This file is commonly used to run batch and script files before running the command that would have originally be run by the Procfile.
procfile.Additional information can be found here: About Procfiles
Example Procfile
web: myscript.bat