Hi @JojenReed,
Could you please share a screenshot of your flow's configuration?
Do you want to change the Created By property within a SharePoint list using Microsoft Flow?
I agree with @Pieter_Veenstra's thought almost. The Created By field is a system field, it would be populated by SharePoint automatically. Changing the Created By field of a SharePoint list is not supported in Microsoft Flow currently, if you would like this feature to be added in Microsoft Flow, please submit an idea to Flow Ideas Forum:
https://powerusers.microsoft.com/t5/Flow-Ideas/idb-p/FlowIdeas
The value filled within Created By field of SharePoint list is based on connections. In other words, if user A create a connection to "Create item" action of SharePoint connector, when the flow run successfully, the Created By field would be populated with User A. If user B create a connection to "Create item" action of SharePoint connector, when the flow run successfully, the Created By field would be populated with User B.
You could consider take a try to create a custom Person or Group type column called Responder in your SharePoint list. Then set the value of this column to Responder's Email dynamic content within "Create item" action:
Best regards,
Kris