Hey,
the system does that automatically for you.
the _owner field gets populated with the logged-in user’s ID whenever a new record is inserted to a collection.
so if your form lets a user submit data to the workout collection, any new entry there will have the logged-in user’s ID in _owner.
you can view the data in the content manager (make sure to display the usually-hidden system fields such as _id, _owner, etc).
hope this helps.