basic functionality, untested.

extension/options.js

@@ -0,0 +1,15 @@
+/* global document */
+
+import browser from 'webextension-polyfill'
+
+document.getElementById('privateKeyInput').addEventListener('input', ev => {
+  browser.storage.local
+    .set({private_key: document.getElementById('privateKeyInput').value})
+    .then(() => {
+      console.log('success')
+    })
+})
+
+browser.storage.local.get('private_key').then(results => {
+  document.getElementById('privateKeyInput').value = results.private_key
+})