asp.net mvc 4 - How to pass the javascript value while submitting the form? -

i new asp.net mvc4. want pass java script value when submitting form. want pass value of random field in javascript along text box values. please me. in advance. javascript code generating , validating captcha:

<script type="text/javascript">     var captchastring = '';     function getcaptcha() {         var chars = "0aa1bb2cc3dd4ee5ff6gg7hh8ii9jj0kk1ll2mm3nn4oo5pp6qq7rr8ss9tt0uu1vv2ww3xx4yy5zz";         var string_length = 7;         captchastring = '';         (var = 0; < string_length; i++) {             var rnum = math.floor(math.random() * chars.length);             captchastring += chars.substring(rnum, rnum + 1);         }         document.getelementbyid("randomfield").innerhtml = captchastring;         document.getelementbyid("forgot_password_form").submit();     }      function validation() {         var text2 = document.getelementbyid("txtcode").value;         if (text2 == captchastring) {              var email = document.getelementbyid("useremail").value;              x = document.getelementbyid("demo");             // find element              x.innerhtml = "valid";             x.style.color = "#ff0000";          }         else {              x = document.getelementbyid("demo");  // find element             x.innerhtml = "invalid captcha. try again";             x.style.color = "#ff0000";             return false;         }     }   </script> 

code submiting form:

    @using (html.beginform("forgotpassword", "login", formmethod.post, new { @class="forgot_password_form"}))      {         @html.textboxfor(u => u.useremail, new { @class = "form-control", placeholder = "email" }) <input type="text" name="txtcaptcha" id="txtcaptchaid"  class="form-control" placeholder="enter code here" />     <input type="submit" class="btn btn-success btn-block" style="float: right; width:30%;" value="reset password" onclick="validation()" /> <p id="demo"> </p>       }