Disabling custom keyboards for specific text fields

Question

I am trying to disconnect a user from using their own keyboards in one of my text fields. When conducting research, it turned out that to disable user keyboards, I need the following code:

func application(application: UIApplication, shouldAllowExtensionPointIdentifier extensionPointIdentifier: String) -> Bool {
    if extensionPointIdentifier == UIApplicationKeyboardExtensionPointIdentifier {
        return false
    }
    return true
}

However, I would not want to disable the keyboard for all my text fields. How can I apply this only to a specific text field? Thank!