Email notification sending for custom workflow in d365

Hi people,

I have to custom workflow provided by a ISV provider. It can send notifications to user who submitted the Workflow. Now, They want to send notifications to user who created the record. for example, they have created custom workflow for customer creation and they want to send the email notification to the user who created the customer along with the user who submitted the workflow. 

I am trying to create a post event handler to completed method in the workflow classes. But, not sure how to go ahead.

Please let me know if you need any further clarification.

Thanks in Advance