Comment on Issue or Pull Request, asking @all-contributors to add a contributor:
@all-contributors please add <username> for <contributions>
See the Emoji Key (Contribution Types Reference) for a list of valid contribution types.
The bot will then create a Pull Request to add the contributor, then reply with the pull request details.
Your request to the bot doesn't need to be perfect. The bot will use basic Natural Language Parsing to determine your intent. For example, this will work too:
Jane you are crushing it in documentation and your infrastructure work has been great too. Let's add jane.doe23 for her contributions. cc @all-contributors