If you’ve heroku toolbelt:
If you’re using the Heroku Toolbelt, the newer syntax is
heroku git:remote -a project
See this for more.
Credits: user101289‘s solution
Else if you don’t have heroku toolbelt:
First do this:
git remote add heroku git@heroku.com:{heroku-app-name}.git
Then do this:
git push heroku master
heroku open