There's no button to add a server in Minecraft Bedrock on Xbox. But it is there on PC and iOS.

Steps to Reproduce:

1. To recreate simply go into Minecraft on Xbox

2. Click Play 

3. Go all the way right to servers 

4. Scroll down the right of to the bottom of featured servers

Observed Results:

You will see no button to add a server 

You will see a slight border of where the button should be but you can't scroll down anymore. 

Expected Results:
You should be able to see a button to add an external server.