$wsproxy = New-WebServiceProxy -Uri $wsdl

$wsproxy.Credentials.Username = "Username"
 $wsproxy.Credentials.Password = "password"

<soapenv:Header>
  <wsse:Security xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
     xmlns:wsse="http://docs.oasisopen.xmlns:wsu="http://docs.oasisopen.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd"
     soapenv:mustUnderstand="0">
  <wsse:UsernameToken>
    <wsse:Username>[user@domain.com]</wsse:Username>
    <wsse:Password>[password]</wsse:Password>
  </wsse:UsernameToken>
 </wsse:Security>
</soapenv:Header>