This option works only with an onscreen keyboard. If you have an external keyboard attached to your device, it’s a no-go unless you open the onscreen keyboard.
As you type a website address or email adress, when you get to the suffix portion(.net, .com. org, etc.), tap and hold the period key. A white bar will appear above the key and it contains the following domain suffixes: .us, .edu, .com, .net, and .org. The .com suffix is selected by default, but you can slide your finger across the bar to select the other options.
If you're using an external keyboard, you probably have a bubble at the bottom of your screen with keyboard and microphone icons. you can tap the keyboard, then tap Show keyboard and then perform the above steps. But obviously, you could have typed the domain suffix much faster.