using arc4random to generate random number to add to string for UI testing login xcode UI testing

I am testing sign up on an app, using xcode UI testing i am wondering how to use the arc4random to generate a random number which will be typed into the app?

here is my code but getting a type (string) Void has no subscript members error.

 let emailBox = self.app.textFields["Email"]
    if emailBox.exists {enter code here
        emailBox.tap()
        emailBox.typeText["testing" + arc4random()]
    }