I want to select a customer in invoice with customer email. What should be the process to achieve that?

By script I’m sur about that and maybe by workflow ( you need to test it)

Workflow and script will do. The email must be unique though which is not guaranteed for customers. Corporates may use a same generic email for different entities. You will get a result but might invoice a wrong entity if customer emails are not unique.

How can I do it with workflow? what would be the action/