Calling a rfc function module with a dynamic field

Hi,

I wish to call a function module from an external system, namely

HR_GETEMPLOYEEDATA_FROMUSER and i wish to default the

user name field to the user currently logged in to SAP i.e. sy-uname.

Is there a way to pass the value in to the fm so that the call

runs with the value of username = sy-uname.

i.e. what should ??? be in the following call from the remote system :

CALL FUNCTION 'HR_GETEMPLOYEEDATA_FROMUSER'

EXPORTING

username = ???

....

Hope you can help! Many thanks in advance!

Samir