To request a certificate you need to submit a Pull Request, this can be done completely via the GitHub web interface:

1. Click "Create new file"
2. Name the file "APP_ID/APP_ID.csr" where APP_ID is your app's id
3. Paste the contents of your CSR file into text area below
4. Commit the file and open a new pull request
5. Nice to have: a link to the source code of the new app

You don't need to mention anybody. People who can help here are subscribed to this repo and will come back.