based on http://add0n.com/useragent-switcher.html#IDComment1054680296

data/options/index.html

@@ -5,6 +5,10 @@
   <style>
     body { padding: 10px; }
     textarea { width: 100%; }
+    #custom {
+      white-space: nowrap;
+      overflow: auto;
+    }
   </style>
 </head>
 
@@ -24,13 +28,22 @@
       </td>
     </tr>
     <tr>
-      <td><textarea id="whitelist" rows="3"></textarea></td>
+      <td><textarea id="whitelist" rows="3" placeholder="e.g.: www.google.com, www.bing.com"></textarea></td>
+    </tr>
+    <tr>
+      <td>
+        <label><input type="radio" name="mode" value="custom" id="mode-custom"> Custom mode: Only apply user-agent string to the tabs with a matching top-level hostnames.</label> <a href="#" id="sample">Insert</a> a sample.
+      </td>
+    </tr>
+    <tr>
+      <td><textarea id="custom" rows="5"></textarea></td>
     </tr>
     <tr>
       <td><label><input type="checkbox" id="faqs"> Open FAQs page on updates</label></td>
     </tr>
   </table>
   <p>
+    <button id="donate">Support Development</button>
     <button id="save">Save</button>
     <span id="status"></span>
   </p>

data/options/index.js

@@ -1,4 +1,13 @@
 'use strict';
+
+function notify(msg) {
+  // Update status to let user know options were saved.
+  const status = document.getElementById('status');
+  status.textContent = msg;
+  clearTimeout(notify.id);
+  notify.id = setTimeout(() => status.textContent = '', 750);
+}
+
 function prepare(str) {
   return str.split(/\s*,\s*/)
   .map(s => s.replace('http://', '')
@@ -7,17 +16,23 @@ function prepare(str) {
 }
 
 function save() {
+  let custom = {};
+  try {
+    custom = JSON.parse(document.getElementById('custom').value);
+  }
+  catch (e) {
+    notify(e.message);
+  }
+
   chrome.storage.local.set({
     faqs: document.getElementById('faqs').checked,
     blacklist: prepare(document.getElementById('blacklist').value),
     whitelist: prepare(document.getElementById('whitelist').value),
-    mode: document.getElementById('mode-blacklist').checked ? 'blacklist' : 'whitelist'
+    custom,
+    mode: document.querySelector('[name="mode"]:checked').value
   }, () => {
     restore();
-    // Update status to let user know options were saved.
-    const status = document.getElementById('status');
-    status.textContent = 'Options saved.';
-    setTimeout(() => status.textContent = '', 750);
+    notify('Options saved.');
   });
 }
 
@@ -26,14 +41,28 @@ function restore() {
     faqs: true,
     mode: 'blacklist',
     whitelist: [],
-    blacklist: []
+    blacklist: [],
+    custom: {}
   }, prefs => {
     document.getElementById('faqs').checked = prefs.faqs;
-    document.getElementById('mode-blacklist').checked = prefs.mode === 'blacklist';
-    document.getElementById('mode-whitelist').checked = prefs.mode === 'whitelist';
+    document.querySelector(`[name="mode"][value="${prefs.mode}"`).checked = true;
     document.getElementById('blacklist').value = prefs.blacklist.join(', ');
     document.getElementById('whitelist').value = prefs.whitelist.join(', ');
+    document.getElementById('custom').value = JSON.stringify(prefs.custom, null, 2);
   });
 }
 document.addEventListener('DOMContentLoaded', restore);
 document.getElementById('save').addEventListener('click', save);
+
+document.getElementById('sample').addEventListener('click', () => {
+  document.getElementById('custom').value = JSON.stringify({
+    'www.google.com': 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.84 Safari/537.36',
+    'www.bing.com': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:57.0) Gecko/20100101 Firefox/57.0'
+  }, null, 2);
+});
+
+document.getElementById('donate').addEventListener('click', () => {
+  chrome.tabs.create({
+    url: 'https://www.paypal.me/addondonation/10usd'
+  });
+});