Set Variable when uploading WF in Server

We have a WF that needs a Fixed Variable value to be set when the file is uploaded to the server.
Instead of creating a process in PS changing the value manually and then generate the import file, could this be set when the file is being uploaded or when a tenant is being assigned to execute the WF.
We do not want to use the credential for this matter. A similar function like credential for a single variable that can be hidden would be great.
Any ideas