Assigning steps to the form submitter
If the form submitted doesn't have a WordPress user account please checkout the doc on assigning steps to an email field (link below).
It's possible to assign steps and emails to the WordPress user account form submitter only if the form is set up to require users to be logged in.
First, in the form setting check the setting to require the user to be logged in.
Now the 'Created by' field will be available in the 'Assign to' field.
It will also be available in the email settings for the approval and user input steps.