Short answer
HubSpot's KB: select Set to repeat, then choose a number and days, weeks, months or years. Repeated tasks keep the same title, assignee, notes and associations. An assigned Core, Sales or Service seat is required to create, use or be assigned recurring tasks.
1. The option
A new task in my demo portal, not saved.
2. Developers
Recurring tasks have their own task series API in the sources.