@marcus … you are correct that the documentation does not mention registration_url or join_url … but in my testing, they are returned (one or the other depending on the meeting type).
In my testing, a type 2 meeting returns registration_url, see below.
But please note I have found another problem with the API when you use it to create a meeting requiring registration (type 1 and 0) - this may be your root problem and why registration_url is not returned. Double-check your response. (Note that for the meeting below I used the API to create it, but then manually edited the meeting settings via the admin website panel to make registration required.)
My observation is that new meeting API calls requesting approval types 0 & 1 always result in a new meeting being created – but always with approval type 2, a non-registration open meeting. See discussion and code here.
{
“uuid”: “redacted”,
“id”: redacted,
“host_id”: “redacted”,
“topic”: “Test API Meeting - one device”,
"type": 2, 
“status”: “waiting”,
“start_time”: “2020-06-15T15:00:00Z”,
“duration”: 60,
“timezone”: “America/Los_Angeles”,
“agenda”: “My agenda goes here”,
“created_at”: “2020-06-11T19:25:32Z”,
“start_url”: “https://us02web.zoom.us/s/redacted”,
“join_url”: “https://us02web.zoom.us/j/redacted”,
"registration_url": “https://us02web.zoom.us/meeting/register/redacted”,
“password”: “redacted”,
“h323_password”: “redacted”,
“pstn_password”: “redacted”,
“encrypted_password”: “redacted”,
“settings”: {
“host_video”: true,
“participant_video”: true,
“cn_meeting”: false,
“in_meeting”: false,
“join_before_host”: true,
“mute_upon_entry”: false,
“watermark”: false,
“use_pmi”: false,
“approval_type”: 0,
“audio”: “both”,
“auto_recording”: “none”,
“enforce_login”: false,
“enforce_login_domains”: “”,
“alternative_hosts”: “”,
“close_registration”: false,
“registrants_confirmation_email”: true,
“waiting_room”: true,
“global_dial_in_countries”: [
“US”
],
“global_dial_in_numbers”: [
{
redacted
}
],
“contact_name”: “redacted”,
“contact_email”: “redacted”,
“registrants_email_notification”: true,
“meeting_authentication”: false
}
}