UIWebView *webView = [[UIWebView alloc] initWithFrame:CGRectZero];

    NSString *userAgent = [webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];

    NSString *newUserAgent = [userAgent stringByAppendingString:@"native_iOS"];//自定义需要拼接的字符串

    NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:newUserAgent, @"UserAgent", nil];

    [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];

__block WKWebView* web = [[WKWebView alloc]initWithFrame:CGRectZero];

    [web evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id result, NSError *error) {

        NSString *userAgent = result;

        NSString *newUserAgent = [userAgent stringByAppendingString:@"/native_iOS"];//自定义需要拼接的字符串

        NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:newUserAgent, @"UserAgent", nil];

        [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];

        //如果是web是局部变量需要一些操作来延长它的生命周期，如下这样写就可以

        web = nil;

    }];