Short answer
HubSpot's developer docs: POST to the sequences enrollments endpoint with your user ID, and include sequenceId, contactId and senderEmail, which must be an email connected to HubSpot. The user needs a Sales or Service Hub Professional or Enterprise seat, and modification endpoints need automation.sequences.enrollments.write.
1. The request
POST /automation/sequences/2026-09/enrollments?userId=YOUR_USER_ID
{ "sequenceId": "123", "contactId": "456", "senderEmail": "you@yourcompany.com" }The path uses the dated API version shown in the guide; check it for the current version.