I needed to run a specific npm script on my app in pm2 (for each env) In my case, it was when I created a staging/test service
The command that worked for me (the args must be forwarded that way):
pm2 start npm --name "my-app-name" -- run "npm:script"
examples:
pm2 start npm --name "myApp" -- run "start:test"
pm2 start npm --name "myApp" -- run "start:staging"
pm2 start npm --name "myApp" -- run "start:production"
Hope it helped