How to add Headers to a request ?

Hi,

I'm currently using SOAP Ui 3.0.1 (not the pro version) and I'm facing an issue.

When I generate a new request by clicking on an operation (then 'New Request'), the second line contains this : <soapenv:Header/>

How can I tell to SOAPUi to insert my tags here ?

I'd like to see for example :

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:xxx">
   <soapenv:Header>
      <MyTag>XXX</MyTag>
   </soapenv:Header>
   <soapenv:Body>
       ...
   </soapenv:Body>
</soapenv:Envelope>

Is it possible with the WSDL file ?
Thx