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I need to send an email to the user who created a transaction, how can I select that field in a Workflow? (the email of the "Created By" user)

I think I was assuming that you're sending it to the person as they're creating the record lol. On second thought, that might not be what you're after. If you're running a transaction approval workflow, I might have it set a custom "created by" field to the creators email and then use that in whatever you're trying to do here.

Yes create a custom field on the transaction type = list 
List/record = employees
Set default value to current user
Then use that field in your workflow. Just as Justin pointed out accept do 
From field
Current record
(Custom field you made)