Hi @anthonys123 , the only field in Dataverse table msdyn_projecttasks that seems to relate to the actual person who completed the task is 'Owner'. Created_By and Modified_By both seem to store 'Microsoft Project' (presumably the owner of the process updating the task).
On a side note, have you tried to insert a new row into msdyn_project table (either manually, or using a flow)? It always seems to fail for me, presumably as some keys are invalid, but the errors don't tell me which key is failing so it's not helpful at all.
Kind regards,
Ben.