From my knowledge, you cannot do this as a user input. However, you can call the logged on user from within a command in the workflow. The Powershell commandlet would be Get-WfaRestParameter "userName" . A good reference command would be the 'Send email' Command. That Command has a couple different calls that may be handy.
Essentially, if you wanted you could make the 'username' field optional. Then in the command if the 'username' user input variable is null or empty, you can use the logged on user.