how to make soap request and get the response using asp.net

Question

im trying to make a webrequest using wsdl, aacheck webservice contains 3 parameter username,password,presultdesc_out the third one should be the response to the request

so i have added the url from VS2015 and its just seems fine 

when i go to the code i have make and object of my service reference 

the code below 

ServiceReference1.nwebserviceClient req = new ServiceReference1.nwebserviceClient();
        req.aaCheck(username.Text, password.Text,result.Text);

the error on the third parameter which display

"arguemnt 3 must be passed with ref keyword "

what should i do ? how to fix it ?