$ heroku config:set MONGODB_URI="my url";
in last part of mongodb Atlas url there is "?retryWrites=true&w=majority"
and Heroku is telling 'w' is not recognized as an internal or external command, operable program or batch file.
What I must do, I am getting Heroku application error;
If you set this environment variable via Heroku dasboard it will work. Go to https://dashboard.heroku.com/apps/ click your app, go to Settings --> Reveal Config Vars and add your variable.