Can't deploy to Heroku

[dlapz1001]

I used the Deploy To Heroku button in the Readme. The log shows it was built successfully, and it was deployed, and both dynos are on. But if I try to go to the app the page just says Bad Request (400) and the log on heroku shows the message

2018-12-04T02:32:25.003239+00:00 app[web.1]: 02:32:25 - ERROR [django.security.DisallowedHost] 
request_id=0bc30a9d-00aa-4a19-9044-67c54d41712b Invalid HTTP_HOST header: 
'organizertest12345.herokuapp.com'. You may need to add u'organizertest12345.herokuapp.com' to 
ALLOWED_HOSTS.

2018-12-04T02:32:25.881019+00:00 heroku[router]: at=info method=GET path="/" 
host=organizertest12345.herokuapp.com request_id=0bc30a9d-00aa-4a19-9044-67c54d41712b 
fwd="47.20.104.252" dyno=web.1 connect=4ms service=890ms status=400 bytes=199 protocol=https

2018-12-04T02:32:26.170876+00:00 app[web.1]: 02:32:26 - ERROR [django.security.DisallowedHost] 
request_id=d8cc1c5b-1840-456b-a02c-9eca0bb44c1a Invalid HTTP_HOST header: 
'organizertest12345.herokuapp.com'. You may need to add u'organizertest12345.herokuapp.com' to 
ALLOWED_HOSTS.

2018-12-04T02:32:26.460568+00:00 heroku[router]: at=info method=GET path="/favicon.ico" 
host=organizertest12345.herokuapp.com request_id=d8cc1c5b-1840-456b-a02c-9eca0bb44c1a 
fwd="47.20.104.252" dyno=web.1 connect=4ms service=317ms status=400 bytes=199 protocol=https

Am I not doing something right? How would I add the URL to the ALLOWED_HOSTS ?