Insert a header at the top of the output XML. This code inserts the static header <?xml version="1.0" ?> in the output XML file that is identified by the fileref MYOUTPUT.

filename myoutput "u:\out.xml";
proc metadata
     header=simple
     out=myoutput
     in="<GetTypes>
         <Types/>
         <Ns>SAS</Ns>
         <Flags/>
         <Options/>
         </GetTypes>";
run;