<script type="text/javascript">
function save() {
  var username = document.getElementById('username').value;
  var password = document.getElementById('password').value;
  chrome.storage.local.set({'username': username}, function(){});
  chrome.storage.local.set({'password': password}, function(){});
}
</script>
Roll Number
<input type="text" name="username" id="username"></input>
Password
<input type="password" name="password" id="password"></input>
<button onclick="save()" type="submit" name="submit">Save</button>

{
  "manifest_version": 2,

  "name": "Autologin",
  "description": "Blah bhla.",
  "version": "1.0",
  "permissions": [
    "http://xxxxxxxxxxxxx/*",
    "storage"
  ],
  "content_scripts": [
    {
      "matches": ["http://xxxxxxxxxxxxx/*"],
      "js": ["autologin.js"]
    }
  ],
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "autologin.html"
  }
}

function save() {
  var username = document.getElementById('username').value;
  var password = document.getElementById('password').value;
  chrome.storage.local.set({
   'username': username ,
   'password': password
  }, function(){});

}

document.getElementByName("submit").onclick=save;

"default_popup": "popup.html" //the name of your popup html file