You've to use first a logon call to your instance, like this:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:xtk:session">
<soapenv:Header/>
<soapenv:Body>
<urn:Logon>
<urn:sessiontoken></urn:sessiontoken>
<urn:strLogin>xxxxxx</urn:strLogin>
<urn:strPassword>xxxxxxx</urn:strPassword>
<urn:elemParameters></urn:elemParameters>
</urn:Logon>
</soapenv:Body>
</soapenv:Envelope>And with SOAPAction=xtk:session#Logon as header.
it will give you a session token, which you have to use in your actual call, between sessiontoken brackets. Remember to change the header, now it must be SOAPAction=nms:rtEvent#PushEvents
If this fails, check instance url, instance users and that you're using a technical operator or one with enough rights
