How can I add a link for a rate button with swift?

Try This, change appId in your method by your App ID

Swift 5

import StoreKit

func rateApp() {
    if #available(iOS 10.3, *) {
        SKStoreReviewController.requestReview()

    } else if let url = URL(string: "itms-apps://itunes.apple.com/app/" + "appId") {
        if #available(iOS 10, *) {
            UIApplication.shared.open(url, options: [:], completionHandler: nil)

        } else {
            UIApplication.shared.openURL(url)
        }
    }
}

Swift 3 \ 4

func rateApp() {
    guard let url = URL(string: "itms-apps://itunes.apple.com/app/" + "appId") else {
        return
    }
    if #available(iOS 10, *) {
        UIApplication.shared.open(url, options: [:], completionHandler: nil)

    } else {
        UIApplication.shared.openURL(url)
    }
}

id959379869 This is the id when you go on your Itune page of your app

Example :
https://itunes.apple.com/fr/app/hipster-moustache/id959379869?mt=8

How get the ID :

1. Itunesconnect account
2. My Apps
3. Click on “+” Button
4. New iOS App
5. Fill require details
6. After filling all details goto your App
7. Click on More Button
8. View on AppStore
9. It will redirect you to your App URL this will be universal
10. Look Http URL