如果要设置User-Agent HTTP标头对于将用于Web视图加载的请求，

If you want to set User-Agent HTTP header for your request that is going to be used for Web-view loading,

let userAgent = "Custom User Agent";
let myURL = NSURL(string: "http://http://web-example")
let myURLRequest:NSURLRequest = NSMutableURLRequest(URL: myURL!)
myWbView.loadRequest(myURLRequest)    
myURLRequest.setValue(userAgent, forHTTPHeaderField: "User-Agent")

如果你想要为您应用中的所有请求设置User-Agent，请参阅此问题
如何设置用户代理？ Swift中UIWebView的标题

If you want to set User-Agent for all requests in your app, see this question How can I set the "User-Agent" header of a UIWebView in Swift