There's a typo in OOlashyn's post, the process Action is "Send Email Confirmation To Contact" . By default the Send Email in the Action is configured to send from the Owner of the Contact (portal user) record, if that owing user does not have Email configured then it will fail. You can set the sender to another user or a queue that has email enabled (personally I tend to use queues for sending outbound emails for the portal).
In the make.powerapps interface you will find the Action under Processes.
