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