[Res]How to add a variable and a value to a request?

I wish to send a request which contains a static value (number 3) and a variable (userID of request) in the request. How can i concat (join) the 2 values?

The attempts below do not work.
<locationId>"3" & ${startNewOptionalPlateApplication#Request#declare namespace apo='http://transport.wa.gov.au/apo-ws/'; //apo:startNewOptionalPlateApplication[1]/startNewOptionalPlateApplicationRequest[1]/userId[1]}</locationId>

<locationId>"3" + ${startNewOptionalPlateApplication#Request#declare namespace apo='http://transport.wa.gov.au/apo-ws/'; //apo:startNewOptionalPlateApplication[1]/startNewOptionalPlateApplicationRequest[1]/userId[1]}</locationId>


<locationId>3 + ${startNewOptionalPlateApplication#Request#declare namespace apo='http://transport.wa.gov.au/apo-ws/'; //apo:startNewOptionalPlateApplication[1]/startNewOptionalPlateApplicationRequest[1]/userId[1]}</locationId>

<locationId>'3' & ${startNewOptionalPlateApplication#Request#declare namespace apo='http://transport.wa.gov.au/apo-ws/'; //apo:startNewOptionalPlateApplication[1]/startNewOptionalPlateApplicationRequest[1]/userId[1]}</locationId>

Thanks.

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:apo="http://transport.wa.gov.au/apo-ws/">
   <soapenv:Header/>
   <soapenv:Body>
      <apo:startNewOptionalPlateApplication>
         <!--Optional:-->
         <startNewOptionalPlateApplicationRequest>
            <userId>3</userId>
            <locationId>3</locationId>