add Sample code for Customized UserAgent

Signed-off-by: Nlixiang <lixiang380@huawei.com>

zh-cn/application-dev/reference/apis/js-apis-webview.md

@@ -1992,6 +1992,37 @@ struct WebComponent {
 }
 ```
 
+支持开发者基于默认的UserAgent去定制UserAgent。
+```ts
+// xxx.ets
+import web_webview from '@ohos.web.webview';
+
+@Entry
+@Component
+struct WebComponent {
+  controller: web_webview.WebviewController = new web_webview.WebviewController();
+  @State ua: string = ""
+
+  aboutToAppear():void {
+    web_webview.once('webInited', () => {
+      try {
+        // 应用侧用法示例，定制UserAgent。
+        this.ua = this.controller.getUserAgent() + 'xxx';
+      } catch(error) {
+        console.error(`ErrorCode: ${error.code},  Message: ${error.message}`);
+      }
+    })
+  }
+
+  build() {
+    Column() {
+      Web({ src: 'www.example.com', controller: this.controller })
+        .userAgent(this.ua)
+    }
+  }
+}
+```
+
 ### getTitle
 
 getTitle(): string