All commands need to be made in a PR review comment. Just start the comment with the "/command", and the results will be sent back to the same PR as a new comment. Note that you cannot use /commands in the regular PR comments, because PullApprove currently doesn't receive events for those (nuance of the GitHub App API).
This command will use the rest of the comment body (optionally wrapped in a
markdown code block) and use it as the
.pullapprove.yml. It won't actually
make any changes to the PR (like request reviewers or update the status), but it
will return the results of what would have happened so that you can see how
the config you tested might work differently than the one on your default