This is done through the Output Settings. In the Email tab, you would enter the following in the To or CC field:
where field_name is the name of the email field as defined in the Designer
Note: If you are pulling an email address from the document to use as a recipient for an email, you will need to make this field mandatory and add a pattern match validation rule to ensure that the entry is a valid email address. Otherwise, the email delivery will fail if this field is left blank or an invalid email address is entered (i.e. "none").
You could use a company email address as the default for this field. That way, any document that is not sent directly to this user-entered email address can be handled as needed. For example: Customer does not have an email address. Default email is set up as firstname.lastname@example.org. The person monitoring this email address will know that the invoice will need to be sent through regular mail instead.