To request a certificate you need to submit a Pull Request, this can be done completely via the GitHub web interface:
- Click "Create new file"
- Name the file "APP_ID/APP_ID.csr" where APP_ID is your app's id
- Paste the contents of your CSR file into text area below
- Commit the file and open a new pull request
- Mention @MorrisJobke
- Nice to have: a link to the source code of the new app