Do 'push' from gitlab to heroku or use discord webhooks in heroku

I'm creating a discord bot (in javascript) and I would like for each update to use the webhook for discord (heroku I do not get the url from discord's webhook), try it with github, which is an option that has heroku, but can not be put in private for free, then I was looking for a lot of information on the webhooks of / in heroku and nothing, then I saw that gitlab is also an option, that brings webhooks and put the project in private, now it works, but my problem now is that I would prefer to optimize this process by doing that in the same way that the project is uploaded to gitlab, it is instantly (or almost) boosted to heroku. Then I have several questions:

• How do I automatically upload the project in javascript from gitlab to heroku?

• Could you run the discord url (webhook) in the webhooks of heroku?

• (only to inform but if anyone knows I will be grateful) is
  can you do git push -u (dos sitios[heroku & gitlab, por ejemplo]) master ?