I hope this might help to someone. I found a method on how to add a git submodule and auto deploy it on a specific folder on my site.
on the plesk panel. I navigated on the websites & domains tab, and on the dev tools, I chose Git:
then, I clicked create new repository. Input the github url, and configured the folder path on where I would like to deploy it:
after creating the new repository, it would automatically clone the repo on that path.
to allow plesk to be notified when certain events happen and automatically clone the changes, pull, and deploy from the github repo. I used webhook feature. I edited the git repository in plesk panel, and copied the webhook url:
after that, I navigated to the my github repository and clicked settings then webhooks:
added a webhook and add the webhook url you just copied in plesk:
...and tadaa!!
hope it helps!