How to deploy an app to pipelines in heroku?

Question

I followed the documentation here https://devcenter.heroku.com/articles/pipelines and setup a pipeline with two stages:

The above link only talks about how to promote an app, without specifying how to deploy, this section specifically (we can read beyond this point and see there is no instruction on how to deploy):

Pipelines let you define how your deployed code flows from one environment to the next. For example, you can deploy code to your staging app (which builds it into a slug) and later promote that same slug to production. This promotion flow ensures that production contains the exact same code that you tested in staging, and it’s also much faster than rebuilding the slug.

So I used the same command we use for deploying non-pipeline based apps (as instructed here):

docker push registry.heroku.com/ultimate-hello-world-cra-stage/web

This appears to be a wrong approach. You can see in snapshot above that STAGING app did get deployed, but when I try to promote the app using this command heroku pipelines:promote --app ultimate-hello-world-cra-stage, I get this error: Error: Pipeline promotions are not supported on apps pushed via Heroku.

What is the correct way to deploy to a pipeline and where can I find it in the docs?