Short answer
HubSpot's KB: add an automation card under Enroll in Workflow to records, then enroll from the record. The workflow must match the object type. Records enroll even if they don't meet the triggers.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Name cards clearly so reps pick the right workflow.
3. Related
Enrollment triggers and unenrolling.