We have the need to have a massive list of recipients with custom fields, is it possible to send a campaing only to a specific set of people based on custom fields through the API?
The POST body to create a campain requires a list ID and I found nothing that suggest to create a campain and send it to a specific segment
You can send a campaign to just a segment, or a number of different segments. All you need to do is specify the segment when saving the draft campaign. If you are sending to a segment only, then you do not need to specify the ListID, you can leave that out.
Check out the documentation on creating a draft campaign here: https://www.campaignmonitor.com/api/campaigns/#creating_a_draft_campaign
Hope that helps!