test.go

package main

import (
    "encoding/xml"
    "fmt"
)

const data = `<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
   <soapenv:Header>
      <ns:credentials>
         <ns:login>
            <ns:username>asdasd</ns:username>
            <ns:password>asdasd</ns:password>
         </ns:login>
      </ns:credentials>
   </soapenv:Header>
   <soapenv:Body>
      <ns:getSessionId>
         <ns:username>weqwe</ns:username>
         <ns:password>qwe</ns:password>
      </ns:getSessionId>
   </soapenv:Body>
</soapenv:Envelope>`

type getSessionId struct {
    XMLName xml.Name
}

type Body struct {
    XMLName xml.Name
    getSessionId getSessionId `xml:"getSessionId"`
}

type Envelope struct {
    XMLName xml.Name
    Body Body `xml:"Body"`
}

func main() {
    var envelope Envelope
    err := xml.Unmarshal([]byte(data), &envelope)
    fmt.Println(envelope, err)
}