Best way to execute post deploy script on Heroku

Question

To run my app on heroku I use pm2, and I configure my Procfile with:

worker: node main.js

Before start my app, I need to configure same modules of pm2 with script

npm i -g pm2

pm2 install pm2-logrotate
pm2 install pm2-slack

pm2 set pm2-slack:slack_url $SLACK_URL
pm2 set pm2-slack:log true
pm2 set pm2-slack:error true
pm2 set pm2-slack:restart true
pm2 set pm2-slack:exist true
pm2 set pm2-slack:start true
pm2 set pm2-slack:online true
pm2 set pm2-slack:stop true

So, where put this scripts?

Tried

I try to put in postinstall script on package.json

  .... 
  "scripts": {
    "start": "pm2 start ecosystem.json",
    "postinstall": "sh post-install.sh"
  },

When I loggin into heroku run bash

λ heroku run bash
Running bash on <heroku-server>... up, run.XXX
~ $ pm2 l

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                        -------------

[PM2] Spawning PM2 daemon
[PM2] PM2 Successfully daemonized
┌──────────┬────┬──────┬─────┬────────┬─────────┬────────┬────────┬──────────┐
│ App name │ id │ mode │ pid │ status │ restart │ uptime │ memory │ watching │
└──────────┴────┴──────┴─────┴────────┴─────────┴────────┴────────┴──────────┘
 Use `pm2 show <id|name>` to get more details about an app

Questions

1. My Procfile was not executed? Why?
2. Where the best place to put mu postinstall script? Should I create my own buildpack and put here?