Set placeholder text to "Port" instead so it is clear what is needed here

index.html

@@ -64,7 +64,7 @@
                   <label class="control-label" for="host">WeeChat relay hostname and port number</label>
                   <div class="input-group">
                     <input type="text" class="form-control monospace" id="host" ng-model="host" placeholder="Address">
-                    <input type="text" class="form-control monospace" id="port" ng-model="port" placeholder="9001">
+                    <input type="text" class="form-control monospace" id="port" ng-model="port" placeholder="Port">
                   </div>
                   <label class="control-label" for="password">WeeChat relay password</label>
                   <input type="password" class="form-control monospace" id="password" ng-model="password" placeholder="Password">