Define a workflow to create an approval task and select the approver during execution time

I would like to create a workflow that when a file is uploaded into the folder it generates an approval task, and during the execution of the workflow to be able to select the approver. I was trying to define a workflow as such, but the Approver field is required during the design of the workflow and I would need to select the approvers dynamically. Please advise how it can be done. Thanks!