A task can be created by a from when it is setup under Actions in the Form Builder.
In the Web Portal, go to Forms > Existing Templates > Edit Arrow for existing or + New Form Template > Actions > Select Create CRM Task from the Action Type Drop Down > Fill in the details > select Save Actions.
Once this has been set up, then any new forms created with the template will create a new Task when the form is submitted.
- What are Form Actions?