How to define XML namespaces to be incuded in the header of XML request which will be send via invoke of a Web Service call?

I have developed a C# Class in VS 2012 from which I have to call (consume) through https, a remote web service method. I have already apply code to create custom headers for security tag, however I must apply in the root a declaration of namespace like

xmlns:ns1="http://ift.tt/LRW8Ij"

The complete XML request that must be send through web service method invoke will be ( and has been tested successfully with SOAP UI ) as of the following :

<?xml version="1.0" encoding="UTF-8"?>
<env:Envelope xmlns:env="http://ift.tt/sVJIaE"
    xmlns:xsi="http://ift.tt/ra1lAU"
    xmlns:xsd="http://ift.tt/tphNwY" xmlns:ns1="http://ift.tt/LRW8Ij">
    <env:Header>
        <ns1:Security>
            <ns1:UsernameToken>
                <ns1:Username>XXXXXXXXXXXXXXXXXXXX</ns1:Username>
                <ns1:Password>ZZZZZZZZZZZZZZZZZZZZ</ns1:Password>
            </ns1:UsernameToken>
        </ns1:Security>
    </env:Header>
    <env:Body>
        <ns:vhWsVersion/>
    </env:Body>
</env:Envelope>

for this, to work the namespace

xmlns:ns1="http://ift.tt/LRW8Ij"

must be include via the invoke of web service method.

Any assistance of how to achieve this is kindly appreciated

Thanx J.