NSString *newAgent = @" Theatre";
WKWebViewConfiguration *webViewConfig = [[WKWebViewConfiguration alloc] init];
webViewConfig.allowsInlineMediaPlayback = YES;
NSString *userAgent = webViewConfig.applicationNameForUserAgent;
userAgent = [userAgent stringByAppendingString:@"myapp/1.0.0"];
webViewConfig.applicationNameForUserAgent = userAgent;
[self.dwebview evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id _Nullable result, NSError * _Nullable error) {
        if (error) {
            NSLog(@"UA: Error == %@", error.localizedDescription);
        } else {
            NSLog(@"userAgent == %@", result);
            NSString *userAgent=result;
            NSString *newUserAgent = [userAgent stringByAppendingString:newAgent];
            NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:newUserAgent, @"UserAgent", nil];
            [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];
            [[NSUserDefaults standardUserDefaults] synchronize];
            
            if (ISIPHONE) {
                
            }else{
                [self.dwebview setCustomUserAgent:newUserAgent];
            }
            if (@available(iOS 9.0, *)) {
                     [self.dwebview setCustomUserAgent:newUserAgent];
                 } else {
                     [self.dwebview setValue:newUserAgent forKey:@"applicationNameForUserAgent"];
                  }
        }
        NSString *urlString =self.apiLinkURL;
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:urlString]];
        [self.dwebview loadRequest:request];
   }];

必须在加载网页之前设置userAgent。