Passwordless bad connection universal login new

This is Django code, but likely clear enough so that it makes sense to read as pseudo-code:

def login(request):
    params = {
        'client_id': settings.AUTH0_CLIENT_ID,
        'client_secret': settings.AUTH0_CLIENT_SECRET,
        'connection': 'sms',
    }
    login_url = requests.Request(
        'POST',
        f'https://{settings.AUTH0_DOMAIN}/passwordless/start',
        params=params,
    ).prepare().url
    return redirect(login_url)

Which returns

    "error": {
      "message": "missing client parameter",
      "oauthError": "invalid_request",
      "type": "request-error"
    },

in the logs, and

{"error":"bad.connection","error_description":"Missing required property: connection"}

in the browser.

Maybe I’m doing this wrong, but what I want to do is, from my regular web app, click a button that launches the Universal Login, and does the entire SMS send/code response flow on your servers, and then returns to me through a callback with a code/token that says “this user is authenticated”. What am I doing wrong if that’s my goal?

I am trying to do this entirely over phone and SMS. No email. JUST Passwordless sms